Unix time doesn’t help with timezones… It’s always in UTC.
Unix timestamps also get a bit weird because of leap seconds. Unix timestamps have no support for leap seconds (the POSIX spec says a Unix day is always exactly 86400 seconds), so they’re usually implemented by repeating the same timestamp twice. This means that the timestamp is ambiguous for that repeated second - one timestamp actually refers to two different moments in time. To quote the example from Wikipedia:
Unix time numbers are repeated in the second immediately following a positive leap second. The Unix time number 1483142400 is thus ambiguous: it can refer either to start of the leap second (2016-12-31 23:59:60) or the end of it, one second later (2017-01-01 00:00:00). In the theoretical case when a negative leap second occurs, no ambiguity is caused, but instead there is a range of Unix time numbers that do not refer to any point in UTC time at all.
Some systems instead spread a positive leap second across the entire day (making each second a very very tiny bit longer) but technically this violates POSIX since it’s modifying the length of a second.
Aren’t timestamps fun?
Luckily, the standards body that deals with leap seconds has said they’ll be discontinued by 2035, so at least it’s one less thing that developers dealing with timestamps will have to worry about.
Don’t try to write your own date/time code. Just don’t. Use something built by someone else.