[zowie_vd]

In short, Unix time is based on UTC. If there's a leap second at midnight, you start a stopwatch ten seconds before midnight and stop it ten seconds after, then there is going to be a one second difference between your stopwatch and the difference between the Unix time timestamps of the moments you started and stopped the stopwatch.

[rand_flip_bit]

For the purposes of a stopwatch/timer, this is exactly what you want. A measurement of time that is exact and does not include leap seconds. Accounting for DST or leap seconds or any other changes to the local timezone for that matter would be disastrous. Imagine you are trying to bake a cake and the timer doesn’t go off for an extra hour due to the clocks getting set back. Now you just have a ruined cake :(

As far as Unix and UTC are concerned, many systems provide the option to smear the system clock over a 24h period (-12h, +12h) centered on the leap second. This avoids the discontinuity and can help avoid errors some incorrectly written programs may not be able to tolerate.

For leap smearing, see: https://developers.google.com/time/smear