Definitions of date arithmetic

[D. J. Bernstein]

Ken Pizzini writes:
> This, to compute the date six months and three days before date X:

Could you explain exactly what you mean by that?

What is 1 month before 31 March? What is 1 month before 16 March? What
is 1 month before 1 March?

What is 1 month after 1 February? What is 1 month after 16 February?
What is 1 month after 28 February?

> Now the API itself needs a fair bit more thought than I've given it;

Semantics first, please.

---Dan
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