Timespan translateToUTC problematic

Previous Topic Next Topic
 
classic Classic list List threaded Threaded
26 messages Options
12
Reply | Threaded
Open this post in threaded view
|

Timespan translateToUTC problematic

dario trussardi
Ciao,

        i have a Pharo 4.0 image run on macbook with time zone is set to Rome +1 UTC .

        When i send the translateToUTC to Date instance it is ok:

                Date today >  2017-11-16T00:00:00+01:00

                Date today translateToUTC > 2017-11-16T00:00:00+00:00

       
        But for Month instance :

                Month month:  6 year:  2015 > 2015-06-01T00:00:00+01:00

                ( Month month:  6 year:  2015 )  translateToUTC > 2015-06-01T00:00:00+01:00

        The same problematic is for Year instance.

        Considerations ?

        Thanks,

                Dario
Reply | Threaded
Open this post in threaded view
|

Re: Timespan translateToUTC problematic

Pavel Krivanek-3

2017-11-16 18:36 GMT+01:00 Trussardi Dario Romano <[hidden email]>:
Ciao,

        i have a Pharo 4.0 image run on macbook with time zone is set to Rome +1 UTC .

        When i send the translateToUTC to Date instance it is ok:

                Date today      >  2017-11-16T00:00:00+01:00

                Date today translateToUTC       >       2017-11-16T00:00:00+00:00


        But for Month instance :

                Month month:  6 year:  2015     >       2015-06-01T00:00:00+01:00

                ( Month month:  6 year:  2015 )  translateToUTC >       2015-06-01T00:00:00+01:00

        The same problematic is for Year instance.

        Considerations ?

        Thanks,

                Dario

Reply | Threaded
Open this post in threaded view
|

Re: Timespan translateToUTC problematic

Peter Uhnák
I find the concept of translating TZ of a Date silly. The real bug imho should be that it prints both time and TZ.... this is Date, not DateAndTime.

Peter

On Thu, Nov 16, 2017 at 8:23 PM, Pavel Krivanek <[hidden email]> wrote:

2017-11-16 18:36 GMT+01:00 Trussardi Dario Romano <[hidden email]>:
Ciao,

        i have a Pharo 4.0 image run on macbook with time zone is set to Rome +1 UTC .

        When i send the translateToUTC to Date instance it is ok:

                Date today      >  2017-11-16T00:00:00+01:00

                Date today translateToUTC       >       2017-11-16T00:00:00+00:00


        But for Month instance :

                Month month:  6 year:  2015     >       2015-06-01T00:00:00+01:00

                ( Month month:  6 year:  2015 )  translateToUTC >       2015-06-01T00:00:00+01:00

        The same problematic is for Year instance.

        Considerations ?

        Thanks,

                Dario


Reply | Threaded
Open this post in threaded view
|

Re: Timespan translateToUTC problematic

Sven Van Caekenberghe-2
In reply to this post by dario trussardi
Hmm, in Pharo 7 I see this:

TimeZone local.

=> a LocalTimeZone(LT1:00)

{
 Date today.
 Date today translateToUTC.
 Month month: 6 year: 2015.
 (Month month: 6 year: 2015) translateToUTC } collect: [ :each | each printString -> each start ].

=>

an Array(
 '17 November 2017'->2017-11-17T00:00:00+00:00
 '17 November 2017'->2017-11-17T00:00:00+00:00
 'June 2015'->2015-06-01T00:00:00+00:00
 'June 2015'->2015-06-01T00:00:00+00:00)

Seems correct to me. And the internal start DateAndTime is already in UTC.

> On 16 Nov 2017, at 18:36, Trussardi Dario Romano <[hidden email]> wrote:
>
> Ciao,
>
> i have a Pharo 4.0 image run on macbook with time zone is set to Rome +1 UTC .
>
> When i send the translateToUTC to Date instance it is ok:
>
> Date today >  2017-11-16T00:00:00+01:00
>
> Date today translateToUTC > 2017-11-16T00:00:00+00:00
>
>
> But for Month instance :
>
> Month month:  6 year:  2015 > 2015-06-01T00:00:00+01:00
>
> ( Month month:  6 year:  2015 )  translateToUTC > 2015-06-01T00:00:00+01:00
>
> The same problematic is for Year instance.
>
> Considerations ?
>
> Thanks,
>
> Dario


Reply | Threaded
Open this post in threaded view
|

Re: Timespan translateToUTC problematic

Prof. Andrew P. Black
In reply to this post by Peter Uhnák

On 17 Nov 2017, at 08:49 , Peter Uhnák <[hidden email]> wrote:

I find the concept of translating TZ of a Date silly. The real bug imho should be that it prints both time and TZ.... this is Date, not DateAndTime.


I live in Oregon, and frequently work with people in New Zealand, which is (depending on the time of year) 19 to 21 hours ahead of Oregon.  So, when it is 3pm at home, it is noon the next day in New Zealand.

This means that in order to know the day of the week, the month, and even the year, of a given instant in UTC, one has to know the timezone that is being referred to.  The answer could be Sunday, 31 December 2017 for one observer, and Monday, 1 January 2018 for another.

So, contrary to your statement, I don’t see how I can know the date without also knowing the Time Zone.

Andrew
Reply | Threaded
Open this post in threaded view
|

Re: Timespan translateToUTC problematic

Peter Uhnák
Because Date has, by definition, no concept of time.
The only reason why you see it is because someone decided to save a bit of time and reuse implementation.

If you want to move TZ, you need something that _has_ time. Such as DateAndTime.

You can also read the comments ...

Date
> Instances of Date are Timespans with duration of 1 day.

it represents an entire day, not a particular time point

DateAndTime
> I represent a point in time or timestamp as defined by ISO 8601. 
> I am TimeZone aware. 

I really don't understand why are you trying to force TZ into Date against it's purpose when you have a class that does exactly what you want and was built for that purpose.

Peter

On Fri, Nov 17, 2017 at 12:09 PM, Prof. Andrew P. Black <[hidden email]> wrote:

On 17 Nov 2017, at 08:49 , Peter Uhnák <[hidden email]> wrote:

I find the concept of translating TZ of a Date silly. The real bug imho should be that it prints both time and TZ.... this is Date, not DateAndTime.


I live in Oregon, and frequently work with people in New Zealand, which is (depending on the time of year) 19 to 21 hours ahead of Oregon.  So, when it is 3pm at home, it is noon the next day in New Zealand.

This means that in order to know the day of the week, the month, and even the year, of a given instant in UTC, one has to know the timezone that is being referred to.  The answer could be Sunday, 31 December 2017 for one observer, and Monday, 1 January 2018 for another.

So, contrary to your statement, I don’t see how I can know the date without also knowing the Time Zone.

Andrew

Reply | Threaded
Open this post in threaded view
|

Re: Timespan translateToUTC problematic

Sven Van Caekenberghe-2
Both interpretation are correct and valid, it is just hard to capture them with one class.

In normal human conversation and in the abstract, of course a date is just a year/month/date triple. But that is because most people only look at this from their own perspective. However, from an international perspective, of course it must include time zone info. Remember end-of-year/new-year, with all those articles about people in other countries partying or having fireworks earlier/later.

Date transitions are different in each time zone, so to talk accurately about a date, you need the context of a time zone.

Whether that timezone is actually part of the date object is another discussion, but there is always the timezone context even if it is implicit ('of course I mean my own timezone and not yours').

> On 17 Nov 2017, at 14:19, Peter Uhnák <[hidden email]> wrote:
>
> Because Date has, by definition, no concept of time.
> The only reason why you see it is because someone decided to save a bit of time and reuse implementation.
>
> If you want to move TZ, you need something that _has_ time. Such as DateAndTime.
>
> You can also read the comments ...
>
> Date
> > Instances of Date are Timespans with duration of 1 day.
>
> it represents an entire day, not a particular time point
>
> DateAndTime
> > I represent a point in time or timestamp as defined by ISO 8601.
> > I am TimeZone aware.
>
> I really don't understand why are you trying to force TZ into Date against it's purpose when you have a class that does exactly what you want and was built for that purpose.
>
> Peter
>
> On Fri, Nov 17, 2017 at 12:09 PM, Prof. Andrew P. Black <[hidden email]> wrote:
>
>> On 17 Nov 2017, at 08:49 , Peter Uhnák <[hidden email]> wrote:
>>
>> I find the concept of translating TZ of a Date silly. The real bug imho should be that it prints both time and TZ.... this is Date, not DateAndTime.
>>
>
> I live in Oregon, and frequently work with people in New Zealand, which is (depending on the time of year) 19 to 21 hours ahead of Oregon.  So, when it is 3pm at home, it is noon the next day in New Zealand.
>
> This means that in order to know the day of the week, the month, and even the year, of a given instant in UTC, one has to know the timezone that is being referred to.  The answer could be Sunday, 31 December 2017 for one observer, and Monday, 1 January 2018 for another.
>
> So, contrary to your statement, I don’t see how I can know the date without also knowing the Time Zone.
>
> Andrew
>


Reply | Threaded
Open this post in threaded view
|

Re: Timespan translateToUTC problematic

Alistair Grant
On 17 November 2017 at 14:44, Sven Van Caekenberghe <[hidden email]> wrote:
> Both interpretation are correct and valid, it is just hard to capture them with one class.
>
> In normal human conversation and in the abstract, of course a date is just a year/month/date triple. But that is because most people only look at this from their own perspective. However, from an international perspective, of course it must include time zone info. Remember end-of-year/new-year, with all those articles about people in other countries partying or having fireworks earlier/later.
>
> Date transitions are different in each time zone, so to talk accurately about a date, you need the context of a time zone.

I don't think this is correct.

If we take CEST (UTC+1) and AEDT (UTC+11) as an example: From midnight
to 14:00 UTC the dates are the same.  From 14:00 - 24:00 CEST
Australia is one day ahead.

How can this be interpreted sensibly given only a date?  Unless I'm
missing something, I think Peter is correct and that a Date shouldn't
have any concept of timezone.

Cheers,
Alistair


> Whether that timezone is actually part of the date object is another discussion, but there is always the timezone context even if it is implicit ('of course I mean my own timezone and not yours').
>
>> On 17 Nov 2017, at 14:19, Peter Uhnák <[hidden email]> wrote:
>>
>> Because Date has, by definition, no concept of time.
>> The only reason why you see it is because someone decided to save a bit of time and reuse implementation.
>>
>> If you want to move TZ, you need something that _has_ time. Such as DateAndTime.
>>
>> You can also read the comments ...
>>
>> Date
>> > Instances of Date are Timespans with duration of 1 day.
>>
>> it represents an entire day, not a particular time point
>>
>> DateAndTime
>> > I represent a point in time or timestamp as defined by ISO 8601.
>> > I am TimeZone aware.
>>
>> I really don't understand why are you trying to force TZ into Date against it's purpose when you have a class that does exactly what you want and was built for that purpose.
>>
>> Peter
>>
>> On Fri, Nov 17, 2017 at 12:09 PM, Prof. Andrew P. Black <[hidden email]> wrote:
>>
>>> On 17 Nov 2017, at 08:49 , Peter Uhnák <[hidden email]> wrote:
>>>
>>> I find the concept of translating TZ of a Date silly. The real bug imho should be that it prints both time and TZ.... this is Date, not DateAndTime.
>>>
>>
>> I live in Oregon, and frequently work with people in New Zealand, which is (depending on the time of year) 19 to 21 hours ahead of Oregon.  So, when it is 3pm at home, it is noon the next day in New Zealand.
>>
>> This means that in order to know the day of the week, the month, and even the year, of a given instant in UTC, one has to know the timezone that is being referred to.  The answer could be Sunday, 31 December 2017 for one observer, and Monday, 1 January 2018 for another.
>>
>> So, contrary to your statement, I don’t see how I can know the date without also knowing the Time Zone.
>>
>>       Andrew
>>
>
>

Reply | Threaded
Open this post in threaded view
|

Re: Timespan translateToUTC problematic

Sven Van Caekenberghe-2


> On 18 Nov 2017, at 17:46, Alistair Grant <[hidden email]> wrote:
>
> On 17 November 2017 at 14:44, Sven Van Caekenberghe <[hidden email]> wrote:
>> Both interpretation are correct and valid, it is just hard to capture them with one class.
>>
>> In normal human conversation and in the abstract, of course a date is just a year/month/date triple. But that is because most people only look at this from their own perspective. However, from an international perspective, of course it must include time zone info. Remember end-of-year/new-year, with all those articles about people in other countries partying or having fireworks earlier/later.
>>
>> Date transitions are different in each time zone, so to talk accurately about a date, you need the context of a time zone.
>
> I don't think this is correct.
>
> If we take CEST (UTC+1) and AEDT (UTC+11) as an example: From midnight
> to 14:00 UTC the dates are the same.  From 14:00 - 24:00 CEST
> Australia is one day ahead.
>
> How can this be interpreted sensibly given only a date?  Unless I'm
> missing something, I think Peter is correct and that a Date shouldn't
> have any concept of timezone.

Inspect and compare the following two Dates:

{
(DateAndTime fromString: '2018/01/01T00:00:00+11') asDate.
(DateAndTime fromString: '2018/01/01T00:00:00+01') asDate
}.

This is as if you typed 'Date today' at the beginning of next January 1st in your timezone or mine. Both print as '1 January 2018'. In some sense they are equal (the abstract, context free view), but as a timespan [start,stop] interval they are different, they do not include the same points in time. By having the timezone in there, the ambiguity is resolved. That is because the exact start moment is different:

{
(DateAndTime fromString: '2018/01/01T00:00:00+11') asUTC.
(DateAndTime fromString: '2018/01/01T00:00:00+01') asUTC
}.

BTW, my ZTimestamp package is an alternative DateAndTime object that is (1) always in UTC, and thus contains no timezone and (2) has second precision. It is half the size and faster to work with.

By using its accompanying ZTimezone class that loads the Olsen DB, you do the necessary conversions when presenting to humans. That of course then requires the context (current or applicable timezone) to be supplied externally.

(ZTimezone id: 'Australia/Sydney') gmtToLocal: ZTimestamp now.

HTH,

Sven

> Cheers,
> Alistair
>
>
>> Whether that timezone is actually part of the date object is another discussion, but there is always the timezone context even if it is implicit ('of course I mean my own timezone and not yours').
>>
>>> On 17 Nov 2017, at 14:19, Peter Uhnák <[hidden email]> wrote:
>>>
>>> Because Date has, by definition, no concept of time.
>>> The only reason why you see it is because someone decided to save a bit of time and reuse implementation.
>>>
>>> If you want to move TZ, you need something that _has_ time. Such as DateAndTime.
>>>
>>> You can also read the comments ...
>>>
>>> Date
>>>> Instances of Date are Timespans with duration of 1 day.
>>>
>>> it represents an entire day, not a particular time point
>>>
>>> DateAndTime
>>>> I represent a point in time or timestamp as defined by ISO 8601.
>>>> I am TimeZone aware.
>>>
>>> I really don't understand why are you trying to force TZ into Date against it's purpose when you have a class that does exactly what you want and was built for that purpose.
>>>
>>> Peter
>>>
>>> On Fri, Nov 17, 2017 at 12:09 PM, Prof. Andrew P. Black <[hidden email]> wrote:
>>>
>>>> On 17 Nov 2017, at 08:49 , Peter Uhnák <[hidden email]> wrote:
>>>>
>>>> I find the concept of translating TZ of a Date silly. The real bug imho should be that it prints both time and TZ.... this is Date, not DateAndTime.
>>>>
>>>
>>> I live in Oregon, and frequently work with people in New Zealand, which is (depending on the time of year) 19 to 21 hours ahead of Oregon.  So, when it is 3pm at home, it is noon the next day in New Zealand.
>>>
>>> This means that in order to know the day of the week, the month, and even the year, of a given instant in UTC, one has to know the timezone that is being referred to.  The answer could be Sunday, 31 December 2017 for one observer, and Monday, 1 January 2018 for another.
>>>
>>> So, contrary to your statement, I don’t see how I can know the date without also knowing the Time Zone.
>>>
>>>      Andrew
>>>
>>
>>
>


Reply | Threaded
Open this post in threaded view
|

Re: Timespan translateToUTC problematic

Richard A. O'Keefe
In reply to this post by dario trussardi
I think the fundamental question is what a Date is supposed
to represent.  I have spent a LOT of time thinking about date
and time classes over the last 10 years, and have come to the
conclusion that it makes no sense to view a Date as a Timespan.

Let's take an example.
Christmas this year is going to be 2017-12-25 in every country
that uses the Gregorian calendar and observes Christmas at all.
If I ask the question
    "Is Christmas on the same date in Utah as it is in Otago?"
I expect to get the answer YES.
But if I ask the question
    "Is the span of time *called* Christmas day there same
     as the span of time *called* Christmas day here?"
I expect to get the answer NO.

It's not entirely unlike the way that '95 Hanover Street'
is the same street address in my city as '95 Hanover Street'
in Edinburgh, but they correspond to quite different places.
(Here: the Urgent Doctors; there: serviced apartments.)

Returning to Dates, I expect something like
   aDate asTimespan: aTimeZone
to return a timespan that might be 23, 24, or 25 hours
(possibly plus an extra second), with *maybe*
   aDate asTimespan
meaning
   aDate asTimespan: TimeZone here

Naturally the same goes for Weeks, Months, and Years, should
they exist.


Reply | Threaded
Open this post in threaded view
|

Re: Timespan translateToUTC problematic

David T. Lewis
Richard,

That is a very good explanation, and 100% correct.

Dave

On Mon, Nov 20, 2017 at 12:30:38PM +1300, Richard A. O'Keefe wrote:

> I think the fundamental question is what a Date is supposed
> to represent.  I have spent a LOT of time thinking about date
> and time classes over the last 10 years, and have come to the
> conclusion that it makes no sense to view a Date as a Timespan.
>
> Let's take an example.
> Christmas this year is going to be 2017-12-25 in every country
> that uses the Gregorian calendar and observes Christmas at all.
> If I ask the question
>    "Is Christmas on the same date in Utah as it is in Otago?"
> I expect to get the answer YES.
> But if I ask the question
>    "Is the span of time *called* Christmas day there same
>     as the span of time *called* Christmas day here?"
> I expect to get the answer NO.
>
> It's not entirely unlike the way that '95 Hanover Street'
> is the same street address in my city as '95 Hanover Street'
> in Edinburgh, but they correspond to quite different places.
> (Here: the Urgent Doctors; there: serviced apartments.)
>
> Returning to Dates, I expect something like
>   aDate asTimespan: aTimeZone
> to return a timespan that might be 23, 24, or 25 hours
> (possibly plus an extra second), with *maybe*
>   aDate asTimespan
> meaning
>   aDate asTimespan: TimeZone here
>
> Naturally the same goes for Weeks, Months, and Years, should
> they exist.
>

Reply | Threaded
Open this post in threaded view
|

Re: Timespan translateToUTC problematic

Alistair Grant
In reply to this post by Sven Van Caekenberghe-2
On 18 November 2017 at 18:38, Sven Van Caekenberghe <[hidden email]> wrote:

>
>
>> On 18 Nov 2017, at 17:46, Alistair Grant <[hidden email]> wrote:
>>
>> On 17 November 2017 at 14:44, Sven Van Caekenberghe <[hidden email]> wrote:
>>> Both interpretation are correct and valid, it is just hard to capture them with one class.
>>>
>>> In normal human conversation and in the abstract, of course a date is just a year/month/date triple. But that is because most people only look at this from their own perspective. However, from an international perspective, of course it must include time zone info. Remember end-of-year/new-year, with all those articles about people in other countries partying or having fireworks earlier/later.
>>>
>>> Date transitions are different in each time zone, so to talk accurately about a date, you need the context of a time zone.
>>
>> I don't think this is correct.
>>
>> If we take CEST (UTC+1) and AEDT (UTC+11) as an example: From midnight
>> to 14:00 UTC the dates are the same.  From 14:00 - 24:00 CEST
>> Australia is one day ahead.
>>
>> How can this be interpreted sensibly given only a date?  Unless I'm
>> missing something, I think Peter is correct and that a Date shouldn't
>> have any concept of timezone.
>
> Inspect and compare the following two Dates:
>
> {
> (DateAndTime fromString: '2018/01/01T00:00:00+11') asDate.
> (DateAndTime fromString: '2018/01/01T00:00:00+01') asDate
> }.
>
> This is as if you typed 'Date today' at the beginning of next January 1st in your timezone or mine. Both print as '1 January 2018'. In some sense they are equal (the abstract, context free view), but as a timespan [start,stop] interval they are different, they do not include the same points in time. By having the timezone in there, the ambiguity is resolved. That is because the exact start moment is different:
>
> {
> (DateAndTime fromString: '2018/01/01T00:00:00+11') asUTC.
> (DateAndTime fromString: '2018/01/01T00:00:00+01') asUTC
> }.

Sven, thanks for your reply.  I haven't thought about this as much as
Richard, but came to the same conclusion.

I think your comment about being equal "in the abstract, context free
view" gets to the core of the issue.

A date can be abstract, i.e. just a day, month and year, or it can be
a timespan (a 24 hour period starting at a particular timezone).  We
may know the appropriate timezone when we specify the date, or we may
not know until we want to use the date.

As an example, take wishing someone happy birthday.  I want to know
that person's birthday in the abstract.  Each year I will want to
apply it with timezones, i.e. I'll figure out an appropriate time to
contact them given my timezone and their's.  Figuring out when to
contact them certainly doesn't depend on the timezone I was in when I
recorded their  birthday, or the timezone they were in when they were
born.  If I want to know if that person and I have the same birthday,
I won't be taking timezones into account.

Google Calendar has (had?) exactly this problem.  I entered an all day
event as a reminder for someone's birthday.  When the day came I
happened to be in a different timezone and their birthday was from 4pm
one day to 4pm the next.  I don't remember which timezone I was in
when I added the event, and I'm certainly not interested in figuring
it out.  Which of the two dates covered by this "one date" is
the correct one?

It should be easy to convert an abstract date to a concrete date, i.e.
one with a specified timezone, but I think they are two different
concepts.

Thanks!
Alistair




> BTW, my ZTimestamp package is an alternative DateAndTime object that is (1) always in UTC, and thus contains no timezone and (2) has second precision. It is half the size and faster to work with.
>
> By using its accompanying ZTimezone class that loads the Olsen DB, you do the necessary conversions when presenting to humans. That of course then requires the context (current or applicable timezone) to be supplied externally.
>
> (ZTimezone id: 'Australia/Sydney') gmtToLocal: ZTimestamp now.
>
> HTH,
>
> Sven
>
>> Cheers,
>> Alistair
>>
>>
>>> Whether that timezone is actually part of the date object is another discussion, but there is always the timezone context even if it is implicit ('of course I mean my own timezone and not yours').
>>>
>>>> On 17 Nov 2017, at 14:19, Peter Uhnák <[hidden email]> wrote:
>>>>
>>>> Because Date has, by definition, no concept of time.
>>>> The only reason why you see it is because someone decided to save a bit of time and reuse implementation.
>>>>
>>>> If you want to move TZ, you need something that _has_ time. Such as DateAndTime.
>>>>
>>>> You can also read the comments ...
>>>>
>>>> Date
>>>>> Instances of Date are Timespans with duration of 1 day.
>>>>
>>>> it represents an entire day, not a particular time point
>>>>
>>>> DateAndTime
>>>>> I represent a point in time or timestamp as defined by ISO 8601.
>>>>> I am TimeZone aware.
>>>>
>>>> I really don't understand why are you trying to force TZ into Date against it's purpose when you have a class that does exactly what you want and was built for that purpose.
>>>>
>>>> Peter
>>>>
>>>> On Fri, Nov 17, 2017 at 12:09 PM, Prof. Andrew P. Black <[hidden email]> wrote:
>>>>
>>>>> On 17 Nov 2017, at 08:49 , Peter Uhnák <[hidden email]> wrote:
>>>>>
>>>>> I find the concept of translating TZ of a Date silly. The real bug imho should be that it prints both time and TZ.... this is Date, not DateAndTime.
>>>>>
>>>>
>>>> I live in Oregon, and frequently work with people in New Zealand, which is (depending on the time of year) 19 to 21 hours ahead of Oregon.  So, when it is 3pm at home, it is noon the next day in New Zealand.
>>>>
>>>> This means that in order to know the day of the week, the month, and even the year, of a given instant in UTC, one has to know the timezone that is being referred to.  The answer could be Sunday, 31 December 2017 for one observer, and Monday, 1 January 2018 for another.
>>>>
>>>> So, contrary to your statement, I don’t see how I can know the date without also knowing the Time Zone.
>>>>
>>>>      Andrew
>>>>
>>>
>>>
>>
>
>

Reply | Threaded
Open this post in threaded view
|

Re: Timespan translateToUTC problematic

Ben Coman
In reply to this post by David T. Lewis
I don't ponder much on Dates, DateTimes etc, but I just had one interesting image flash through my head 
and was curious how correct it sounds, or if I'm completely off base.

So who remembers the old arcade game "Moon Buggy" ?

Now imagine that passing beneath your wheels is a hyper-fast terrain of DateTime .
Levels are days marked "A" , "B", "C", "D", "E" at *fixed* points on the DateTime terrain, representing when a particular Date begins.

As your day progresses dealing with obstacles in your path the *point* of Date separation approaches you.
In your own timezone you know what Date it is by when your wheels pass over a Date marker.

Now if someone in the US wants to know the date in New Zealand, 
that is like attaching a scanning laser beam to the front of the buggy.  
How far ahead it scans depends on the *difference* between time zones.
As the DateTime terrain passes beneath your wheels, the fixed Date point approaches you, and when it touches your lazer scanner, the Date changes in the other timezone.

So the way to find the date in another timezone, 
is not to give a Date a timezone, but rather to add the difference in timezones to your DateTime 
to get the DateTime in the target country to compare that with timezoneless-Date.


cheers -ben




[1] https://www.youtube.com/watch?v=6EptD9Egf7w

On 20 November 2017 at 10:12, David T. Lewis <[hidden email]> wrote:
Richard,

That is a very good explanation, and 100% correct.

Dave

On Mon, Nov 20, 2017 at 12:30:38PM +1300, Richard A. O'Keefe wrote:
> I think the fundamental question is what a Date is supposed
> to represent.  I have spent a LOT of time thinking about date
> and time classes over the last 10 years, and have come to the
> conclusion that it makes no sense to view a Date as a Timespan.
>
> Let's take an example.
> Christmas this year is going to be 2017-12-25 in every country
> that uses the Gregorian calendar and observes Christmas at all.
> If I ask the question
>    "Is Christmas on the same date in Utah as it is in Otago?"
> I expect to get the answer YES.
> But if I ask the question
>    "Is the span of time *called* Christmas day there same
>     as the span of time *called* Christmas day here?"
> I expect to get the answer NO.
>
> It's not entirely unlike the way that '95 Hanover Street'
> is the same street address in my city as '95 Hanover Street'
> in Edinburgh, but they correspond to quite different places.
> (Here: the Urgent Doctors; there: serviced apartments.)
>
> Returning to Dates, I expect something like
>   aDate asTimespan: aTimeZone
> to return a timespan that might be 23, 24, or 25 hours
> (possibly plus an extra second), with *maybe*
>   aDate asTimespan
> meaning
>   aDate asTimespan: TimeZone here
>
> Naturally the same goes for Weeks, Months, and Years, should
> they exist.
>


Reply | Threaded
Open this post in threaded view
|

Re: Timespan translateToUTC problematic

Ben Coman
In reply to this post by Alistair Grant


On 20 November 2017 at 14:58, Alistair Grant <[hidden email]> wrote:
On 18 November 2017 at 18:38, Sven Van Caekenberghe <[hidden email]> wrote:
>
>
>> On 18 Nov 2017, at 17:46, Alistair Grant <[hidden email]> wrote:
>>
>> On 17 November 2017 at 14:44, Sven Van Caekenberghe <[hidden email]> wrote:
>>> Both interpretation are correct and valid, it is just hard to capture them with one class.
>>>
>>> In normal human conversation and in the abstract, of course a date is just a year/month/date triple. But that is because most people only look at this from their own perspective. However, from an international perspective, of course it must include time zone info. Remember end-of-year/new-year, with all those articles about people in other countries partying or having fireworks earlier/later.
>>>
>>> Date transitions are different in each time zone, so to talk accurately about a date, you need the context of a time zone.
>>
>> I don't think this is correct.
>>
>> If we take CEST (UTC+1) and AEDT (UTC+11) as an example: From midnight
>> to 14:00 UTC the dates are the same.  From 14:00 - 24:00 CEST
>> Australia is one day ahead.
>>
>> How can this be interpreted sensibly given only a date?  Unless I'm
>> missing something, I think Peter is correct and that a Date shouldn't
>> have any concept of timezone.
>
> Inspect and compare the following two Dates:
>
> {
> (DateAndTime fromString: '2018/01/01T00:00:00+11') asDate.
> (DateAndTime fromString: '2018/01/01T00:00:00+01') asDate
> }.
>
> This is as if you typed 'Date today' at the beginning of next January 1st in your timezone or mine. Both print as '1 January 2018'. In some sense they are equal (the abstract, context free view), but as a timespan [start,stop] interval they are different, they do not include the same points in time. By having the timezone in there, the ambiguity is resolved. That is because the exact start moment is different:
>
> {
> (DateAndTime fromString: '2018/01/01T00:00:00+11') asUTC.
> (DateAndTime fromString: '2018/01/01T00:00:00+01') asUTC
> }.

Sven, thanks for your reply.  I haven't thought about this as much as
Richard, but came to the same conclusion.

I think your comment about being equal "in the abstract, context free
view" gets to the core of the issue.

A date can be abstract, i.e. just a day, month and year, or it can be
a timespan (a 24 hour period starting at a particular timezone).  We
may know the appropriate timezone when we specify the date, or we may
not know until we want to use the date.

As an example, take wishing someone happy birthday.  I want to know
that person's birthday in the abstract.  Each year I will want to
apply it with timezones, i.e. I'll figure out an appropriate time to
contact them given my timezone and their's.  Figuring out when to
contact them certainly doesn't depend on the timezone I was in when I
recorded their  birthday, or the timezone they were in when they were
born.  If I want to know if that person and I have the same birthday,
I won't be taking timezones into account.

Nice examples. They made a lot of sense.

 

Google Calendar has (had?) exactly this problem.  I entered an all day
event as a reminder for someone's birthday.  When the day came I
happened to be in a different timezone and their birthday was from 4pm
one day to 4pm the next.  I don't remember which timezone I was in
when I added the event, and I'm certainly not interested in figuring
it out.  Which of the two dates covered by this "one date" is
the correct one?

Obviously GodGoog should know this date was associated with a particular person,
and since they're happy with GG geo-tracking their location, GG should have adapted.
Sounds like a bug to me. 

    \   
;^)-cheers-ben
    /

 

It should be easy to convert an abstract date to a concrete date, i.e.
one with a specified timezone, but I think they are two different
concepts.

Thanks!
Alistair




> BTW, my ZTimestamp package is an alternative DateAndTime object that is (1) always in UTC, and thus contains no timezone and (2) has second precision. It is half the size and faster to work with.
>
> By using its accompanying ZTimezone class that loads the Olsen DB, you do the necessary conversions when presenting to humans. That of course then requires the context (current or applicable timezone) to be supplied externally.
>
> (ZTimezone id: 'Australia/Sydney') gmtToLocal: ZTimestamp now.
>
> HTH,
>
> Sven
>
>> Cheers,
>> Alistair
>>
>>
>>> Whether that timezone is actually part of the date object is another discussion, but there is always the timezone context even if it is implicit ('of course I mean my own timezone and not yours').
>>>
>>>> On 17 Nov 2017, at 14:19, Peter Uhnák <[hidden email]> wrote:
>>>>
>>>> Because Date has, by definition, no concept of time.
>>>> The only reason why you see it is because someone decided to save a bit of time and reuse implementation.
>>>>
>>>> If you want to move TZ, you need something that _has_ time. Such as DateAndTime.
>>>>
>>>> You can also read the comments ...
>>>>
>>>> Date
>>>>> Instances of Date are Timespans with duration of 1 day.
>>>>
>>>> it represents an entire day, not a particular time point
>>>>
>>>> DateAndTime
>>>>> I represent a point in time or timestamp as defined by ISO 8601.
>>>>> I am TimeZone aware.
>>>>
>>>> I really don't understand why are you trying to force TZ into Date against it's purpose when you have a class that does exactly what you want and was built for that purpose.
>>>>
>>>> Peter
>>>>
>>>> On Fri, Nov 17, 2017 at 12:09 PM, Prof. Andrew P. Black <[hidden email]> wrote:
>>>>
>>>>> On 17 Nov 2017, at 08:49 , Peter Uhnák <[hidden email]> wrote:
>>>>>
>>>>> I find the concept of translating TZ of a Date silly. The real bug imho should be that it prints both time and TZ.... this is Date, not DateAndTime.
>>>>>
>>>>
>>>> I live in Oregon, and frequently work with people in New Zealand, which is (depending on the time of year) 19 to 21 hours ahead of Oregon.  So, when it is 3pm at home, it is noon the next day in New Zealand.
>>>>
>>>> This means that in order to know the day of the week, the month, and even the year, of a given instant in UTC, one has to know the timezone that is being referred to.  The answer could be Sunday, 31 December 2017 for one observer, and Monday, 1 January 2018 for another.
>>>>
>>>> So, contrary to your statement, I don’t see how I can know the date without also knowing the Time Zone.
>>>>
>>>>      Andrew
>>>>
>>>
>>>
>>
>
>


Reply | Threaded
Open this post in threaded view
|

Re: Timespan translateToUTC problematic

Alistair Grant
In reply to this post by Ben Coman
On 20 November 2017 at 08:30, Ben Coman <[hidden email]> wrote:

> I don't ponder much on Dates, DateTimes etc, but I just had one interesting
> image flash through my head
> and was curious how correct it sounds, or if I'm completely off base.
>
> So who remembers the old arcade game "Moon Buggy" ?
> https://www.youtube.com/watch?v=6EptD9Egf7w
>
> Now imagine that passing beneath your wheels is a hyper-fast terrain of
> DateTime .
> Levels are days marked "A" , "B", "C", "D", "E" at *fixed* points on the
> DateTime terrain, representing when a particular Date begins.
>
> As your day progresses dealing with obstacles in your path the *point* of
> Date separation approaches you.
> In your own timezone you know what Date it is by when your wheels pass over
> a Date marker.

I'm just glad that Bloc is on the way and we have GTInspector so that
in future when I inspect a Date there'll be a tab there that *you*
wrote with a moon buggy flashing lasers to show me what's happening.
:-)


> Now if someone in the US wants to know the date in New Zealand,
> that is like attaching a scanning laser beam to the front of the buggy.
> How far ahead it scans depends on the *difference* between time zones.
> As the DateTime terrain passes beneath your wheels, the fixed Date point
> approaches you, and when it touches your lazer scanner, the Date changes in
> the other timezone.
>
> So the way to find the date in another timezone,
> is not to give a Date a timezone, but rather to add the difference in
> timezones to your DateTime
> to get the DateTime in the target country to compare that with
> timezoneless-Date.

I think that there's still use for representing a date as a timespan,
i.e. a 24 hour period that we can use to find the intersection of
times, etc (in addition to adding a timezone offset to a DateTime to
figure out the date in a different country).  Whether it needs to be a
separate class from Timespan I'm not so sure.

Cheers,
Alistair


> cheers -ben
>
>
>
>
> [1] https://www.youtube.com/watch?v=6EptD9Egf7w
>
>
> On 20 November 2017 at 10:12, David T. Lewis <[hidden email]> wrote:
>>
>> Richard,
>>
>> That is a very good explanation, and 100% correct.
>>
>> Dave
>>
>> On Mon, Nov 20, 2017 at 12:30:38PM +1300, Richard A. O'Keefe wrote:
>> > I think the fundamental question is what a Date is supposed
>> > to represent.  I have spent a LOT of time thinking about date
>> > and time classes over the last 10 years, and have come to the
>> > conclusion that it makes no sense to view a Date as a Timespan.
>> >
>> > Let's take an example.
>> > Christmas this year is going to be 2017-12-25 in every country
>> > that uses the Gregorian calendar and observes Christmas at all.
>> > If I ask the question
>> >    "Is Christmas on the same date in Utah as it is in Otago?"
>> > I expect to get the answer YES.
>> > But if I ask the question
>> >    "Is the span of time *called* Christmas day there same
>> >     as the span of time *called* Christmas day here?"
>> > I expect to get the answer NO.
>> >
>> > It's not entirely unlike the way that '95 Hanover Street'
>> > is the same street address in my city as '95 Hanover Street'
>> > in Edinburgh, but they correspond to quite different places.
>> > (Here: the Urgent Doctors; there: serviced apartments.)
>> >
>> > Returning to Dates, I expect something like
>> >   aDate asTimespan: aTimeZone
>> > to return a timespan that might be 23, 24, or 25 hours
>> > (possibly plus an extra second), with *maybe*
>> >   aDate asTimespan
>> > meaning
>> >   aDate asTimespan: TimeZone here
>> >
>> > Naturally the same goes for Weeks, Months, and Years, should
>> > they exist.
>> >
>>
>

Reply | Threaded
Open this post in threaded view
|

Re: Timespan translateToUTC problematic

Sven Van Caekenberghe-2
In reply to this post by Alistair Grant


> On 20 Nov 2017, at 07:58, Alistair Grant <[hidden email]> wrote:
>
> On 18 November 2017 at 18:38, Sven Van Caekenberghe <[hidden email]> wrote:
>>
>>
>>> On 18 Nov 2017, at 17:46, Alistair Grant <[hidden email]> wrote:
>>>
>>> On 17 November 2017 at 14:44, Sven Van Caekenberghe <[hidden email]> wrote:
>>>> Both interpretation are correct and valid, it is just hard to capture them with one class.
>>>>
>>>> In normal human conversation and in the abstract, of course a date is just a year/month/date triple. But that is because most people only look at this from their own perspective. However, from an international perspective, of course it must include time zone info. Remember end-of-year/new-year, with all those articles about people in other countries partying or having fireworks earlier/later.
>>>>
>>>> Date transitions are different in each time zone, so to talk accurately about a date, you need the context of a time zone.
>>>
>>> I don't think this is correct.
>>>
>>> If we take CEST (UTC+1) and AEDT (UTC+11) as an example: From midnight
>>> to 14:00 UTC the dates are the same.  From 14:00 - 24:00 CEST
>>> Australia is one day ahead.
>>>
>>> How can this be interpreted sensibly given only a date?  Unless I'm
>>> missing something, I think Peter is correct and that a Date shouldn't
>>> have any concept of timezone.
>>
>> Inspect and compare the following two Dates:
>>
>> {
>> (DateAndTime fromString: '2018/01/01T00:00:00+11') asDate.
>> (DateAndTime fromString: '2018/01/01T00:00:00+01') asDate
>> }.
>>
>> This is as if you typed 'Date today' at the beginning of next January 1st in your timezone or mine. Both print as '1 January 2018'. In some sense they are equal (the abstract, context free view), but as a timespan [start,stop] interval they are different, they do not include the same points in time. By having the timezone in there, the ambiguity is resolved. That is because the exact start moment is different:
>>
>> {
>> (DateAndTime fromString: '2018/01/01T00:00:00+11') asUTC.
>> (DateAndTime fromString: '2018/01/01T00:00:00+01') asUTC
>> }.
>
> Sven, thanks for your reply.  I haven't thought about this as much as
> Richard, but came to the same conclusion.
>
> I think your comment about being equal "in the abstract, context free
> view" gets to the core of the issue.
>
> A date can be abstract, i.e. just a day, month and year, or it can be
> a timespan (a 24 hour period starting at a particular timezone).  We
> may know the appropriate timezone when we specify the date, or we may
> not know until we want to use the date.
>
> As an example, take wishing someone happy birthday.  I want to know
> that person's birthday in the abstract.  Each year I will want to
> apply it with timezones, i.e. I'll figure out an appropriate time to
> contact them given my timezone and their's.  Figuring out when to
> contact them certainly doesn't depend on the timezone I was in when I
> recorded their  birthday, or the timezone they were in when they were
> born.  If I want to know if that person and I have the same birthday,
> I won't be taking timezones into account.

Well, that example is exactly why you do need the TZ (in the date or not is another matter). If I, from my TZ, want to be the first to wish you a happy birthday, I have to know your exact TZ.

We can discuss about this ad infinitum. I think we all agree that there are 2 views (abstract calendar date and concrete time interval/span, which requires a TZ), as well as 2 possible ways to deal with the second case (TZ inside date or as context outside of the date).

Right now, it is TZ inside, but you are free to ignore it. That is how it is, I did not write it, I would probably do it differently myself, but I don't think there is a bug, nor that we have to change it any time soon.

There are several alternative packages out there, and everyone can write their own, maybe one will eventually become the most popular as to be the default in the image, but I doubt it.

> Google Calendar has (had?) exactly this problem.  I entered an all day
> event as a reminder for someone's birthday.  When the day came I
> happened to be in a different timezone and their birthday was from 4pm
> one day to 4pm the next.  I don't remember which timezone I was in
> when I added the event, and I'm certainly not interested in figuring
> it out.  Which of the two dates covered by this "one date" is
> the correct one?
>
> It should be easy to convert an abstract date to a concrete date, i.e.
> one with a specified timezone, but I think they are two different
> concepts.
>
> Thanks!
> Alistair
>
>
>
>
>> BTW, my ZTimestamp package is an alternative DateAndTime object that is (1) always in UTC, and thus contains no timezone and (2) has second precision. It is half the size and faster to work with.
>>
>> By using its accompanying ZTimezone class that loads the Olsen DB, you do the necessary conversions when presenting to humans. That of course then requires the context (current or applicable timezone) to be supplied externally.
>>
>> (ZTimezone id: 'Australia/Sydney') gmtToLocal: ZTimestamp now.
>>
>> HTH,
>>
>> Sven
>>
>>> Cheers,
>>> Alistair
>>>
>>>
>>>> Whether that timezone is actually part of the date object is another discussion, but there is always the timezone context even if it is implicit ('of course I mean my own timezone and not yours').
>>>>
>>>>> On 17 Nov 2017, at 14:19, Peter Uhnák <[hidden email]> wrote:
>>>>>
>>>>> Because Date has, by definition, no concept of time.
>>>>> The only reason why you see it is because someone decided to save a bit of time and reuse implementation.
>>>>>
>>>>> If you want to move TZ, you need something that _has_ time. Such as DateAndTime.
>>>>>
>>>>> You can also read the comments ...
>>>>>
>>>>> Date
>>>>>> Instances of Date are Timespans with duration of 1 day.
>>>>>
>>>>> it represents an entire day, not a particular time point
>>>>>
>>>>> DateAndTime
>>>>>> I represent a point in time or timestamp as defined by ISO 8601.
>>>>>> I am TimeZone aware.
>>>>>
>>>>> I really don't understand why are you trying to force TZ into Date against it's purpose when you have a class that does exactly what you want and was built for that purpose.
>>>>>
>>>>> Peter
>>>>>
>>>>> On Fri, Nov 17, 2017 at 12:09 PM, Prof. Andrew P. Black <[hidden email]> wrote:
>>>>>
>>>>>> On 17 Nov 2017, at 08:49 , Peter Uhnák <[hidden email]> wrote:
>>>>>>
>>>>>> I find the concept of translating TZ of a Date silly. The real bug imho should be that it prints both time and TZ.... this is Date, not DateAndTime.
>>>>>>
>>>>>
>>>>> I live in Oregon, and frequently work with people in New Zealand, which is (depending on the time of year) 19 to 21 hours ahead of Oregon.  So, when it is 3pm at home, it is noon the next day in New Zealand.
>>>>>
>>>>> This means that in order to know the day of the week, the month, and even the year, of a given instant in UTC, one has to know the timezone that is being referred to.  The answer could be Sunday, 31 December 2017 for one observer, and Monday, 1 January 2018 for another.
>>>>>
>>>>> So, contrary to your statement, I don’t see how I can know the date without also knowing the Time Zone.
>>>>>
>>>>>     Andrew


Reply | Threaded
Open this post in threaded view
|

Re: Timespan translateToUTC problematic

Pavel Krivanek-3
In any case, we have an issue because the behavior of the Date and Month is different

-- Pavel

2017-11-20 9:16 GMT+01:00 Sven Van Caekenberghe <[hidden email]>:


> On 20 Nov 2017, at 07:58, Alistair Grant <[hidden email]> wrote:
>
> On 18 November 2017 at 18:38, Sven Van Caekenberghe <[hidden email]> wrote:
>>
>>
>>> On 18 Nov 2017, at 17:46, Alistair Grant <[hidden email]> wrote:
>>>
>>> On 17 November 2017 at 14:44, Sven Van Caekenberghe <[hidden email]> wrote:
>>>> Both interpretation are correct and valid, it is just hard to capture them with one class.
>>>>
>>>> In normal human conversation and in the abstract, of course a date is just a year/month/date triple. But that is because most people only look at this from their own perspective. However, from an international perspective, of course it must include time zone info. Remember end-of-year/new-year, with all those articles about people in other countries partying or having fireworks earlier/later.
>>>>
>>>> Date transitions are different in each time zone, so to talk accurately about a date, you need the context of a time zone.
>>>
>>> I don't think this is correct.
>>>
>>> If we take CEST (UTC+1) and AEDT (UTC+11) as an example: From midnight
>>> to 14:00 UTC the dates are the same.  From 14:00 - 24:00 CEST
>>> Australia is one day ahead.
>>>
>>> How can this be interpreted sensibly given only a date?  Unless I'm
>>> missing something, I think Peter is correct and that a Date shouldn't
>>> have any concept of timezone.
>>
>> Inspect and compare the following two Dates:
>>
>> {
>> (DateAndTime fromString: '2018/01/01T00:00:00+11') asDate.
>> (DateAndTime fromString: '2018/01/01T00:00:00+01') asDate
>> }.
>>
>> This is as if you typed 'Date today' at the beginning of next January 1st in your timezone or mine. Both print as '1 January 2018'. In some sense they are equal (the abstract, context free view), but as a timespan [start,stop] interval they are different, they do not include the same points in time. By having the timezone in there, the ambiguity is resolved. That is because the exact start moment is different:
>>
>> {
>> (DateAndTime fromString: '2018/01/01T00:00:00+11') asUTC.
>> (DateAndTime fromString: '2018/01/01T00:00:00+01') asUTC
>> }.
>
> Sven, thanks for your reply.  I haven't thought about this as much as
> Richard, but came to the same conclusion.
>
> I think your comment about being equal "in the abstract, context free
> view" gets to the core of the issue.
>
> A date can be abstract, i.e. just a day, month and year, or it can be
> a timespan (a 24 hour period starting at a particular timezone).  We
> may know the appropriate timezone when we specify the date, or we may
> not know until we want to use the date.
>
> As an example, take wishing someone happy birthday.  I want to know
> that person's birthday in the abstract.  Each year I will want to
> apply it with timezones, i.e. I'll figure out an appropriate time to
> contact them given my timezone and their's.  Figuring out when to
> contact them certainly doesn't depend on the timezone I was in when I
> recorded their  birthday, or the timezone they were in when they were
> born.  If I want to know if that person and I have the same birthday,
> I won't be taking timezones into account.

Well, that example is exactly why you do need the TZ (in the date or not is another matter). If I, from my TZ, want to be the first to wish you a happy birthday, I have to know your exact TZ.

We can discuss about this ad infinitum. I think we all agree that there are 2 views (abstract calendar date and concrete time interval/span, which requires a TZ), as well as 2 possible ways to deal with the second case (TZ inside date or as context outside of the date).

Right now, it is TZ inside, but you are free to ignore it. That is how it is, I did not write it, I would probably do it differently myself, but I don't think there is a bug, nor that we have to change it any time soon.

There are several alternative packages out there, and everyone can write their own, maybe one will eventually become the most popular as to be the default in the image, but I doubt it.

> Google Calendar has (had?) exactly this problem.  I entered an all day
> event as a reminder for someone's birthday.  When the day came I
> happened to be in a different timezone and their birthday was from 4pm
> one day to 4pm the next.  I don't remember which timezone I was in
> when I added the event, and I'm certainly not interested in figuring
> it out.  Which of the two dates covered by this "one date" is
> the correct one?
>
> It should be easy to convert an abstract date to a concrete date, i.e.
> one with a specified timezone, but I think they are two different
> concepts.
>
> Thanks!
> Alistair
>
>
>
>
>> BTW, my ZTimestamp package is an alternative DateAndTime object that is (1) always in UTC, and thus contains no timezone and (2) has second precision. It is half the size and faster to work with.
>>
>> By using its accompanying ZTimezone class that loads the Olsen DB, you do the necessary conversions when presenting to humans. That of course then requires the context (current or applicable timezone) to be supplied externally.
>>
>> (ZTimezone id: 'Australia/Sydney') gmtToLocal: ZTimestamp now.
>>
>> HTH,
>>
>> Sven
>>
>>> Cheers,
>>> Alistair
>>>
>>>
>>>> Whether that timezone is actually part of the date object is another discussion, but there is always the timezone context even if it is implicit ('of course I mean my own timezone and not yours').
>>>>
>>>>> On 17 Nov 2017, at 14:19, Peter Uhnák <[hidden email]> wrote:
>>>>>
>>>>> Because Date has, by definition, no concept of time.
>>>>> The only reason why you see it is because someone decided to save a bit of time and reuse implementation.
>>>>>
>>>>> If you want to move TZ, you need something that _has_ time. Such as DateAndTime.
>>>>>
>>>>> You can also read the comments ...
>>>>>
>>>>> Date
>>>>>> Instances of Date are Timespans with duration of 1 day.
>>>>>
>>>>> it represents an entire day, not a particular time point
>>>>>
>>>>> DateAndTime
>>>>>> I represent a point in time or timestamp as defined by ISO 8601.
>>>>>> I am TimeZone aware.
>>>>>
>>>>> I really don't understand why are you trying to force TZ into Date against it's purpose when you have a class that does exactly what you want and was built for that purpose.
>>>>>
>>>>> Peter
>>>>>
>>>>> On Fri, Nov 17, 2017 at 12:09 PM, Prof. Andrew P. Black <[hidden email]> wrote:
>>>>>
>>>>>> On 17 Nov 2017, at 08:49 , Peter Uhnák <[hidden email]> wrote:
>>>>>>
>>>>>> I find the concept of translating TZ of a Date silly. The real bug imho should be that it prints both time and TZ.... this is Date, not DateAndTime.
>>>>>>
>>>>>
>>>>> I live in Oregon, and frequently work with people in New Zealand, which is (depending on the time of year) 19 to 21 hours ahead of Oregon.  So, when it is 3pm at home, it is noon the next day in New Zealand.
>>>>>
>>>>> This means that in order to know the day of the week, the month, and even the year, of a given instant in UTC, one has to know the timezone that is being referred to.  The answer could be Sunday, 31 December 2017 for one observer, and Monday, 1 January 2018 for another.
>>>>>
>>>>> So, contrary to your statement, I don’t see how I can know the date without also knowing the Time Zone.
>>>>>
>>>>>     Andrew



Reply | Threaded
Open this post in threaded view
|

Re: Timespan translateToUTC problematic

Alistair Grant
In reply to this post by Sven Van Caekenberghe-2
On 20 November 2017 at 09:16, Sven Van Caekenberghe <[hidden email]> wrote:
>
> We can discuss about this ad infinitum. I think we all agree that
> there are 2 views (abstract calendar date and concrete time
> interval/span, which requires a TZ), as well as 2 possible ways to
> deal with the second case (TZ inside date or as context outside of the
> date).


Right, I think changing things now will be quite a bit of work.

However there are a couple of things we can do to make life a bit easier:

1. Add a method that facilitates TZ independent comparisons.
2. Add a bit more information in the class comments.


I've named the method #dmyEquals: below, although I don't particularly
like it.  If anyone has a better idea, please reply.

I've also added some proposed text for the class comments.

If I don't hear back in a couple of days I'll submit a PR (with
automated tests).


Cheers,
Alistair

----

dmyEquals: aDate
    "Perform a time zone independent comparison of the dates, i.e.
only compare day, month and year"

    ^self year = aDate year and: [
        self monthIndex = aDate monthIndex and:
        [ self dayOfMonth = aDate dayOfMonth ] ]

----
Class comment:

Instances of Date are Timespans with duration of 1 day.
Their default creation assumes a start of midnight in the local time zone.

!Comparing Dates

We tend to use dates in one of two modes:

- Time zone dependent
- Time zone independent

In the first instance, dates are only the same if they are in the same
time zone (otherwise they are two different 24 hour periods).  This is
the default behaviour of Date.

In the second, comparison is only interested in whether the day, month
and year are the same.

As an example, take someone's birthday.  If I want to know whether we
were born on the same day, I will want to compare dates without time
zones.  If I want to know if it is currently their birthday where they
are, I'll want to use time zones.


[[[language=smalltalk
| birthday1 birthday2 |

birthday1 := (DateAndTime fromString: '2018/01/01T00:00:00+10') asDate.
birthday2 := (DateAndTime fromString: '2018/01/01T00:00:00+01') asDate.

"Is it person 1's birthday now? (where they live: UTC+10)"
birthday1 includes: DateAndTime now.

"Do person 1 and person 2 have the same birthday?"
birthday1 dmyEquals: birthday2.
]]]

Reply | Threaded
Open this post in threaded view
|

Re: Timespan translateToUTC problematic

Sven Van Caekenberghe-2


> On 21 Nov 2017, at 18:07, Alistair Grant <[hidden email]> wrote:
>
> On 20 November 2017 at 09:16, Sven Van Caekenberghe <[hidden email]> wrote:
>>
>> We can discuss about this ad infinitum. I think we all agree that
>> there are 2 views (abstract calendar date and concrete time
>> interval/span, which requires a TZ), as well as 2 possible ways to
>> deal with the second case (TZ inside date or as context outside of the
>> date).
>
>
> Right, I think changing things now will be quite a bit of work.
>
> However there are a couple of things we can do to make life a bit easier:
>
> 1. Add a method that facilitates TZ independent comparisons.
> 2. Add a bit more information in the class comments.
>
>
> I've named the method #dmyEquals: below, although I don't particularly
> like it.  If anyone has a better idea, please reply.

#equals:
#calendarEquals:
#equalIgnoreTimezone:

> I've also added some proposed text for the class comments.

More comments are always good.

> If I don't hear back in a couple of days I'll submit a PR (with
> automated tests).
>
>
> Cheers,
> Alistair
>
> ----
>
> dmyEquals: aDate
>    "Perform a time zone independent comparison of the dates, i.e.
> only compare day, month and year"
>
>    ^self year = aDate year and: [
>        self monthIndex = aDate monthIndex and:
>        [ self dayOfMonth = aDate dayOfMonth ] ]
>
> ----
> Class comment:
>
> Instances of Date are Timespans with duration of 1 day.
> Their default creation assumes a start of midnight in the local time zone.
>
> !Comparing Dates
>
> We tend to use dates in one of two modes:
>
> - Time zone dependent
> - Time zone independent
>
> In the first instance, dates are only the same if they are in the same
> time zone (otherwise they are two different 24 hour periods).  This is
> the default behaviour of Date.
>
> In the second, comparison is only interested in whether the day, month
> and year are the same.
>
> As an example, take someone's birthday.  If I want to know whether we
> were born on the same day, I will want to compare dates without time
> zones.  If I want to know if it is currently their birthday where they
> are, I'll want to use time zones.
>
>
> [[[language=smalltalk
> | birthday1 birthday2 |
>
> birthday1 := (DateAndTime fromString: '2018/01/01T00:00:00+10') asDate.
> birthday2 := (DateAndTime fromString: '2018/01/01T00:00:00+01') asDate.
>
> "Is it person 1's birthday now? (where they live: UTC+10)"
> birthday1 includes: DateAndTime now.
>
> "Do person 1 and person 2 have the same birthday?"
> birthday1 dmyEquals: birthday2.
> ]]]
>


Reply | Threaded
Open this post in threaded view
|

Re: Timespan translateToUTC problematic

Pharo Smalltalk Users mailing list
In reply to this post by Alistair Grant
#hasSameDayAs:
 
-----------------
Benoît St-Jean
Yahoo! Messenger: bstjean
Twitter: @BenLeChialeux
Pinterest: benoitstjean
Instagram: Chef_Benito
IRC: lamneth
Blogue: endormitoire.wordpress.com
"A standpoint is an intellectual horizon of radius zero".  (A. Einstein)



From: Alistair Grant <[hidden email]>
To: Any question about pharo is welcome <[hidden email]>
Sent: Tuesday, November 21, 2017 12:07 PM
Subject: Re: [Pharo-users] Timespan translateToUTC problematic

On 20 November 2017 at 09:16, Sven Van Caekenberghe <[hidden email]> wrote:
>
> We can discuss about this ad infinitum. I think we all agree that
> there are 2 views (abstract calendar date and concrete time
> interval/span, which requires a TZ), as well as 2 possible ways to
> deal with the second case (TZ inside date or as context outside of the
> date).


Right, I think changing things now will be quite a bit of work.

However there are a couple of things we can do to make life a bit easier:

1. Add a method that facilitates TZ independent comparisons.
2. Add a bit more information in the class comments.


I've named the method #dmyEquals: below, although I don't particularly
like it.  If anyone has a better idea, please reply.

I've also added some proposed text for the class comments.

If I don't hear back in a couple of days I'll submit a PR (with
automated tests).


Cheers,
Alistair

----

dmyEquals: aDate
    "Perform a time zone independent comparison of the dates, i.e.
only compare day, month and year"

    ^self year = aDate year and: [
        self monthIndex = aDate monthIndex and:
        [ self dayOfMonth = aDate dayOfMonth ] ]

----
Class comment:

Instances of Date are Timespans with duration of 1 day.
Their default creation assumes a start of midnight in the local time zone.

!Comparing Dates

We tend to use dates in one of two modes:

- Time zone dependent
- Time zone independent

In the first instance, dates are only the same if they are in the same
time zone (otherwise they are two different 24 hour periods).  This is
the default behaviour of Date.

In the second, comparison is only interested in whether the day, month
and year are the same.

As an example, take someone's birthday.  If I want to know whether we
were born on the same day, I will want to compare dates without time
zones.  If I want to know if it is currently their birthday where they
are, I'll want to use time zones.


[[[language=smalltalk
| birthday1 birthday2 |

birthday1 := (DateAndTime fromString: '2018/01/01T00:00:00+10') asDate.
birthday2 := (DateAndTime fromString: '2018/01/01T00:00:00+01') asDate.

"Is it person 1's birthday now? (where they live: UTC+10)"
birthday1 includes: DateAndTime now.

"Do person 1 and person 2 have the same birthday?"
birthday1 dmyEquals: birthday2.
]]]



12