Standards for Date/Time addition?

According to the POSIX.1-2001 standard, next month (as in incrementing tm_mon before calling mktime) is done by adjusting the values until they fit. So, for example, next month from January 31, 2001 is March 3, 2001. This is because the tm_mday of 31 isn't valid with tm_mon of 1 (February), so it is normalized to tm_mon of 2 (March) and tm_mday of 3.

The next month from January 31, 2000 is March 2, 2000, because Feb. has 29 days that year. The next month from January, 1 2038 doesn't exist, depending.

The great thing about standards is there are so many to chose from. Check the SQL standard, I bet you can find a different meaning of next month. I suspect ISO 8601 may give you yet another choice. Point is, there are many different behaviors, the meaning of 'next month' is very domain-specific.

edit: I think I've found how SQL-92 handles it, apparently asking for next month from January 31 is an error.

Links:

• SQL-92: http://www.contrib.andrew.cmu.edu/~shadow/sql/sql1992.txt
• POSIX: http://pubs.opengroup.org/onlinepubs/9699919799/ (though apparently that version now defers to ISO C, which doesn't seem as clear. The mktime manpage on my machine is clear, though)
• ISO C: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf
• Java: http://download.oracle.com/javase/6/docs/api/java/util/Calendar.html

I believe the defacto standard is ISO 8601. Unfortunately, there are many ambiguities, for example:

Date arithmetic is not defined

2001-03-30 + P1M = 2001-04-29 (Add 30 days)2001-03-30 + P1M = 2001-04-30 (Add 1 mon.)

Addition is not commutative or associative

2001-03-30 + P1D + P1M = 2001-04-302001-03-30 + P1M + P1D = 2001-05-01

Subtraction is not the inverse of Addition.

Precision of decimal fractions can vary.

The full specification can be found at http://www.iso.org/iso/catalogue_detail.htm?csnumber=26780

I think each product is attempting to adhere to an impossible to implement standard. The ambiguous parts are open to interpretation and so everyone interprets. This is the same standard that opened us up to the Y2K bug!!

Myself, I favor an implementation that converts a date/time to a 1970 based number (UNIX timestamp), performs the calculation and converts back. I believe this is the approach taken by Oracle/MySQL. I am surprised that more attention has not been paid this issue, as it is really important, sometimes critical, in so many applications. Thanks for the question!

Edit: While doing some more reading, I found Joe Celko's thoughts on different date/time representations and standardization HERE.

Query:

SELECTADDDATE(DATE('2010-12-31'), INTERVAL 1 MONTH) 'Dec + Month',ADDDATE(DATE('2011-01-31'), INTERVAL 1 MONTH) 'Jan + Month',ADDDATE(DATE('2011-02-28'), INTERVAL 1 MONTH) 'Feb + Month',ADDDATE(DATE('2011-03-31'), INTERVAL 1 MONTH) 'Mar + Month';

Output:

    Dec + Month  Jan + Month  Feb + Month   Mar + Month    2011-01-31   2011-02-28   2011-03-28    2011-04-30

My conclusion:

1. Calculate the number of days in the month of the input date.
2. Add that many days to the input date.
3. Check if the day in the resulting date exceeds the maximun number of days in the resulting month.
4. If yes, then change the resulting day to maximum day of the resulting month.

If you add MONTH, YEAR_MONTH, or YEAR and the resulting date has a day that is larger than the maximum day for the new month, the day is adjusted to the maximum days in the new month

source

Problem here is that it doesn't mention that the month is actually the month from the input date.