First proposal for new ISO C 9x time API

[D. J. Bernstein]

I should make clear that the design of libtai is focused on the needs of
real programs. Here are the top two uses of timestamps in existing code:

   * Subtraction. ``Tell me exactly how much time this took.''
   * Conversion to civil time. ``The user wants to know when that was.''

Complete support for both of these functions is IMPOSSIBLE if you don't
have an up-to-date leap-second table. (Proof: If you _do_ have both of
those functions, you can easily print an up-to-date leap-second table.)

Markus Kuhn writes:
> My API does not require any leapsecond history to be stored

Then your API is fundamentally broken. How do you expect people to
compute accurate time differences?

> something which I would never expect to be updated reliably in practice.

In theory the necessary information is already transferred by NTP---
although a new protocol would be a good idea for obvious reasons.

---Dan
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