Need advice updating values in todo app

I do not know if seaside or smalltalk question. 

I have now test the todo app and try todo some changes so I understand more seaside + smalltalk.

I have now one new variable in Task object.

I now want it to be updated to 1 month from deadline when I click on "show all tasks" 

Can you give me guidelines how I should do. Or maybe give one small example of the function.

//Dal