[umanwizard]

You’re wrong and have the situation exactly backwards.

If a day has 86,401 or 86,399 seconds due to leap seconds, POSIX time still advances by exactly 86,400.

If you had a perfectly accurate stopwatch running since 1970-01-01 the number it shows now would be different from POSIX time.

[quasarj]

Wait, why would it be different?

[growse]

Unix time is not monatomic. It sometimes goes backwards.

[zokier]

Strictly speaking Unix time is monotonic, because it counts integer number of seconds and it does not go backwards, it only repeats during leap seconds.

[LegionMammal978]

POSIX does define "the amount of time (in seconds and nanoseconds) since the Epoch", for the output of clock_gettime() with CLOCK_REALTIME [0]. That "amount of time" must be stopped or smeared or go backward in some way when it reaches a leap second. This isn't the 80s, we have functions that interact with Unix time at sub-second precision.

[0] https://pubs.opengroup.org/onlinepubs/9799919799/functions/c...

[growse]

This feels like semantics. If a counter repeats a value, it's effectively gone backwards and by definition is not monatomic.

A delta between two monatomic values should always be non-negative. This is not true for Unix time.

[wat10000]

“Monotonic” means non-decreasing (or non-increasing if you’re going the other way). Values are allowed to repeat. The term you’re looking for is “strictly increasing.”

[growse]

I guess this hinges on whether you think Unix time is an integer or a float. If you think it's just an integer, then yes, you can't get a negative delta.

If, however, you think it's a float, then you can.

[umanwizard]

Because a day, that is the time between midnight UTC and midnight UTC, is not always exactly 86400 seconds, due to leap seconds. But Unix time always increases by exactly 86400.