> we do hour offsets twice a year, so if an hour off is okay why is 1 second off not?

The actual Unix time does not change when the clocks are switched between daylight saving time and standard time. Only the time zone changes.

When a leap second occurs, the actual Unix time changes, which can lead to bugs, e.g. when a positive time difference comes back as negative. To prevent such issues, a monotonic clock can be used to measure time intervals.