`Instant::PAST_VALUE` constant?

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Granted that logic is still iffy for Instant::MIN on devices which zero their timers to boot up (but that's probably fine).

This is not a niche thing -- CLOCK_MONOTONIC is counted from boot time on Linux and probably many others. But for the 1 second delay in this example, I agree it's probably fine.

Another quirk of MIN is that this could be far in the past, e.g. on Unix targets that use tv_sec: i64, versus Windows with unsigned Instant(Duration). So the range of Instant::MIN.elapsed() could be wildly different!