Standards for Date/Time addition?

Question

I\'m looking for standards for Date/Time addition. I haven\'t been able to find any. In particular I\'m hoping to find a spec that defines what should happen when you add a mont

Answer 1

Joda-Time in Java chooses the previous valid date when an invalid one is created. For example, 2011-01-31 + P1M = 2011-02-28. I believe this is the most widely chosen default choice in date-time libraries, and thus a de facto standard.

ThreeTen/JSR-310 provides a strategy pattern for this, with four choices, see the code.

More amusing is the question of what the answer to 2011-01-31 + P1M-1D is. If you add the month, then resolve the invalid date, then subtract the day, you get 2011-02-27. But I think most users expect 2011-02-28 because the period is being added in a single lump. See how ThreeTen handles this here.

I have considered trying to write a general purpose best practices in date/time calculations, or actual spec, but haven't really had the time!

Answer 2

First day of the month + 1 month should equal the first of the next month. Trying this on SQL Server

          SELECT CAST ('01/01/2012' AS DateTime), DATEADD (m, 1, '01/01/2012')
UNION ALL SELECT CAST ('02/01/2012' AS DateTime), DATEADD (m, 1, '02/01/2012')
UNION ALL SELECT CAST ('03/01/2012' AS DateTime), DATEADD (m, 1, '03/01/2012')
UNION ALL SELECT CAST ('04/01/2012' AS DateTime), DATEADD (m, 1, '04/01/2012')
UNION ALL SELECT CAST ('05/01/2012' AS DateTime), DATEADD (m, 1, '05/01/2012')

This results in

----------------------- -----------------------
2012-01-01              2012-02-01             
2012-02-01              2012-03-01             
2012-03-01              2012-04-01             
2012-04-01              2012-05-01             
2012-05-01              2012-06-01             

Last day of this month + 1 month should equal last day of next month. This should go for next month, current month, 10 months down, etc.

          SELECT CAST ('01/31/2012' AS DateTime), DATEADD (m, 1, '01/31/2012')
UNION ALL SELECT CAST ('01/30/2012' AS DateTime), DATEADD (m, 1, '01/30/2012')
UNION ALL SELECT CAST ('01/29/2012' AS DateTime), DATEADD (m, 1, '01/29/2012')
UNION ALL SELECT CAST ('01/28/2012' AS DateTime), DATEADD (m, 1, '01/28/2012')
UNION ALL SELECT CAST ('01/27/2012' AS DateTime), DATEADD (m, 1, '01/27/2012')
UNION ALL SELECT CAST ('01/26/2012' AS DateTime), DATEADD (m, 1, '01/26/2012')

This results in

----------------------- -----------------------
2012-01-31              2012-02-29             
2012-01-30              2012-02-29             
2012-01-29              2012-02-29             
2012-01-28              2012-02-28             
2012-01-27              2012-02-27             
2012-01-26              2012-02-26             

See how 31, 30, 29 all become feb 29 (2012 is a leap year).

p.s. I took off the time parts (all zeroes) to help make it more readable