You never do this sort of conversion manually. On linux you can use clock_gettime() with CLOCK_TAI: CLOCK_TAI (since Linux 3.10; Linux-specific)

You never do this sort of conversion manually. On linux you can use clock_gettime() with CLOCK_TAI: CLOCK_TAI (since Linux 3.10; Linux-specific) A nonsettable system-wide clock derived from wall-clock time but ignoring leap seconds. This clock does not experience discontinuities and backwards jumps caused by NTP inserting leap seconds as CLOCK_REALTIME does. The acronym TAI refers to International Atomic Time. Now, leap seconds are advertised in advance and your system know when they occur through a leap seconds file (leap-seconds.list) that needs to be kept up to date.