Definitions of date arithmetic

Paul Eggert eggert at
Sat Oct 10 00:53:16 UTC 1998

   Date: 9 Oct 1998 23:17:26 -0000
   From: "D. J. Bernstein" <djb at>

   What is 1 month before 31 March?

I see two answers to that question:

* ``It's an error to ask that question.''  I.e. the mktime replacement
  would report an error if asked that question.

* 29 February if it's a leap lear, 28 February otherwise.

There are reasonable arguments for either answer.  Perhaps the
standard should leave it open.

A closely related question is ``What is 1 minute before 23:59:60?''.

Here's a good one: ``What is 1 month before 1 February 1995 in
Kiritimati?''.  There was no 1 January 1995 in Kiritimati; they
skipped a day by moving the clocks ahead 24 hours.  Similar issues
arise when asking about times that occur within smaller, more typical
UTC offset changes.  A number of obscure questions arise here, and I'm
not sure it's worth standardizing all the answers.

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