Because that's not how Unix time is defined. The leap second hack is hardcoded into it: Unix time is [...] the number of seconds that have elapsed since the Un

Because that's not how Unix time is defined. The leap second hack is hardcoded into it: Unix time is [...] the number of seconds that have elapsed since the Unix epoch, minus leap seconds. - Wikipedia If you are on Mars you'll have to update your computer time every ~6 months when Earth release a new table of leap seconds caused by e.g. earthquakes.