Functions add/diff/cmp for xtime

[D. J. Bernstein]

Markus Kuhn writes:
> If you are *really* interested in the number of SI seconds between two
> UTC timestamps (very few applications will really need this,

Ever heard of TCP/IP? How about UNIX? How about system logs? Timestamps
in logs aren't just for clock displays; they're also used for profiling
and accounting. Time differences are crucial.

Why do you refuse to admit that clocks are used this way?

> and those usually will better use TIME_MONOTONIC instead of UTC

Do you expect people to record two clocks in their logs? A single TAI
clock is better for everybody.

---Dan
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