RTFM
$ man 2 time
...
POSIX.1 defines seconds since the Epoch as a value to be interpreted as
the number of seconds between a specified time and the Epoch, according
to a formula for conversion from UTC equivalent to conversion on the
naďve basis that leap seconds are ignored and all years divisible by 4
are leap years. This value is not the same as the actual number of
seconds between the time and the Epoch, because of leap seconds and
because clocks are not required to be synchronised to a standard refer-
ence. The intention is that the interpretation of seconds since the
Epoch values be consistent; see POSIX.1 Annex B 2.2.2 for further
rationale.
