Definitions of date arithmetic

[D. J. Bernstein]

Paul Eggert writes:
> * 29 February if it's a leap lear, 28 February otherwise.

If that's 1 month before 31 March, then what's 1 month before 30 March?
1 month before 29 March? 1 month before 28 March?

And what's 1 month after 28 February? 1 month after 27 February? 1 month
_before_ 27 February?

I wouldn't mind adding more date-arithmetic functions to libtai/caldate,
but I haven't seen a coherent explanation of the desired functions.

> * ``It's an error to ask that question.''

I find it difficult to believe that users would be satisfied with that
answer. What applications do you have in mind?

---Dan
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