Time primitive return values

I notice the Time primitives...

    Time class >> millisecondClockValue
        <primitive: 135>
        ^ 0

    Time class >> primUTCMicrosecondsClock
        <primitive: 240>
        self primitiveFailed

return different values on failure.

Not that time is every going to fail ;) ,
but should these be consistent and which is preferable?

cheers -ben