[POSIX] is neither a linear representation of time nor a true representation of UTC […] The Unix time number increases by exactly 86400 each day […] Observe that when a positive leap second occurs (i.e., when a leap second is inserted) the Unix time numbers repeat themselves.
The upside to this is that days are always 86400 seconds "long": computing the start of the next day is simple. (Computing the length is not so much, and computing to-the-second elapsed time is also harder.)
This is somewhat relevant to Python, as the datetime module "ignores" (which I interpret to mean, "repeats the prior second"; I've never watched to see what really happens) leap seconds.
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In POSIX time, seconds can repeat[1],
The upside to this is that days are always 86400 seconds "long": computing the start of the next day is simple. (Computing the length is not so much, and computing to-the-second elapsed time is also harder.)
This is somewhat relevant to Python, as the datetime module "ignores" (which I interpret to mean, "repeats the prior second"; I've never watched to see what really happens) leap seconds.
[1]: https://en.wikipedia.org/wiki/Unix_time#Leap_seconds