Unix time: Measured as the number of seconds since epoch (the beginning of 1970 in UTC). Unix time is not affected by time zones or daylight saving.
I don't think this is strictly correct. This implies that someone could start an atomic stopwatch at midnight on Jan 1, 1970, and it would match Unix time. It won't.
Because Unix time is non-linear and will either slew or repeat seconds when UTC has leap seconds, the hypothetical stopwatch would be ahead of Unix time by 34 seconds.
... at least this is how I understand it. Every time I try and wrap by head around the differences between UTC/TAI/UT1, my head really starts to hurt.
Possibly its a historical accident: UNIX was released before the first leap second, and before anyone could have known they were needed (back when seconds were defined in terms of Earth's movement). Around its release time, the definition of the second was switched (making leap seconds potentially needed). Maybe by the time anyone realized, switching from UTC to TAI would have been too painful?
Not to mention that you can't know the UTC-TAI offset more than a few months into the future. We can not predict which years will have leap seconds inserted.
Unix timestamps do not handle leap seconds well at all. Obvious things like t₂-t₁ fail to provide the number of seconds between t₂ and t₁.
Because people working with timestamps like to write code that says things like:
// schedule another run for tomorrow
schedule_event(now() + 86400)
...which doesn't actually work when leap-seconds are involved (things start to drift by a second). If you specify that leap seconds get replayed, it works.
Strange then that we'd scold a developer for assuming the number of hours in a day, or the timezone, or whatever, but not for using Unix time as an absolute time, when in fact it is not an absolute time at all and even caters to the same error in thought.
I don't think this is strictly correct. This implies that someone could start an atomic stopwatch at midnight on Jan 1, 1970, and it would match Unix time. It won't.
Because Unix time is non-linear and will either slew or repeat seconds when UTC has leap seconds, the hypothetical stopwatch would be ahead of Unix time by 34 seconds.
... at least this is how I understand it. Every time I try and wrap by head around the differences between UTC/TAI/UT1, my head really starts to hurt.