When I used that term in my last comment, I specifically meant the timescale formally defined by POSIX and linked above, which it misleadingly calls "seconds since the Epoch". This is the timescale people usually mean by "Unix time", and it's what UUIDv7 aspires to align to: in its own words, it's "derived from the widely implemented and well-known Unix Epoch timestamp source".

But Unix time isn't a count of SI seconds, as you might wish it to be. Instead, it's effectively "the number of whole days between 1970-01-01 and UTC-today, times 86400, plus the number of SI seconds since the last UTC-midnight." It smashes each UTC day into 86400 'seconds', regardless of whether it is truly longer or shorter due to leap seconds. This makes Unix time non-monotonic at the end of a leap second, since it rolls back that second at the following midnight.

It's nice that the RFC mentions leap seconds at all, but it's really playing with fire to leave it so vague, when monotonicity is an especially important property for these UUIDs. Its brief definition of the UUIDv7 time source as "the number of milliseconds since midnight 1 Jan 1970 UTC, leap seconds excluded" especially doesn't help here, because the discrepancies in UTC are even worse than leap seconds:

> I would say that the instance (or second long interval) in time that we name 2016-12-31T23:59:60Z is 1483228836 seconds after 1970-01-01T00:00:00Z, and that 2017-01-01T00:00:00Z is 1483228837 seconds after 1970-01-01T00:00:00Z [0].

Before 1972, the length of a UTC second was shorter than the length of an SI second. If you account for that, 2016-12-31T23:59:60 UTC is ~1483228827.999918 SI seconds after 1970-01-01T00:00:00 UTC; and 2017-01-01T00:00:00 UTC is ~1483228828.999918 SI seconds after 1970-01-01T00:00:00 UTC. Meanwhile, the "Unix time" (POSIX's "seconds since the Epoch") is 1483228800 for both seconds. (At least, these SI-second intervals are based on the widespread TAI − UTC table [0]. Beware that this table is also inaccurate, in that it was constructed from an older EAL − UTC table using a constant offset, but EAL and TAI continue to run at a different rate due to relativistic effects [1]. Since 1977, EAL − TAI has grown to over 0.00108 SI seconds.)

> I suspect that some of the other commentors in this thread use this version of "seconds" and that some of the confusion/disagreement stems from this difference in definition.

I wish. But the RFC says "leap seconds excluded", and it's designed to align with "Unix time" (which jumps back after every leap second), so clearly it isn't a count of physical SI milliseconds. There's a trilemma here: you can't (a) lie about ("exclude") leap seconds, (b) keep monotonicity, and (c) maintain a constant length of the second, all at the same time; you have to pick two. You yourself would prefer (b) and (c), and POSIX mandates (a) and (c) for "Unix time". But UUIDv7, to align with "Unix time", really wants (a) and (b), which requires smearing, or halting the time scale, or some other dedicated mechanism. Yet the RFC is nearly silent on this.

[0] https://hpiers.obspm.fr/eop-pc/earthor/utc/TAI-UTC_tab.html

[1] https://webtai.bipm.org/ftp/pub/tai/other-products/ealtai/fe...