Notables:
According to POSIX.1, Unix time is supposed to handle a leap second by replaying the previous second
Timezones are a presentation-layer problem! Most of your code shouldn’t be dealing with timezones or local time, it should be passing Unix time around.
Leap seconds happen more often than leap years.
Time passes at a rate of one second per second for every observer. The frequency of a remote clock relative to an observer is affected by velocity and gravity. The clocks inside GPS satellites are adjusted for relativistic effects.
If you want to store a humanly-readable time (e.g. logs), consider storing it along with Unix time, not instead of Unix time.
When displaying time, always include the timezone offset. A time format without an offset is useless.
- The system clock is inaccurate.
- You’re on a network? Every other system’s clock is differently inaccurate.