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Asked: 2026-05-25T21:28:22+00:00

I’m looking for standards for Date/Time addition. I haven’t been able to find any.

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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 month to a date like January 31st. Is the right answer February 28th(/29th)? March 1st? March 2nd?

I’ve seen inconsistent implementations between different tools (PHP & MySQL in this case), and I’m trying to find some sort of standards to base my work on.

Differing Results:

PHP

$end = strtotime("+1 month", 1314835200);
//1317513600   Sat, 01 Oct 2011 20:00:00 -0400

MySQL

SELECT UNIX_TIMESTAMP(DATE_ADD(FROM_UNIXTIME(1314835200), INTERVAL 1 MONTH));
#1317427200    Fri, 30 Sep 2011 20:00:00 -0400

Oracle

SELECT ADD_MONTHS('31-Aug-11', 1) FROM dual;
#30-SEP-11

(sorry for the format change, my oracle foo is weak)

Java

Calendar c = Calendar.getInstance();
c.clear();
c.set( 2011, Calendar.AUGUST, 31 );
c.add( Calendar.MONTH, 1 );
c.getTime()
#Fri Sep 30 00:00:00 EDT 2011
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1 Answer

  1. Editorial Team

    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:

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