Both https://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xbd_chap04.html#tag_21_04_16 https://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xbd... and

Both https://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xbd_chap04.html#tag_21_04_16 https://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xbd... and https://en.wikipedia.org/wiki/Unix_time https://en.wikipedia.org/wiki/Unix_time indicate that leap seconds are not included in Unix time as seconds-since-the-epoch. Leap seconds are included in UTC, and thus the Unix time appears to skip a second relative to UTC.