What are best practices/tips on handling date and time everyone has in general? Every time it is a bit of a nightmare.
- Understand the semantic difference between a timestamp (absolute time) and clock/calendar time (relative time). Understand which one your use case uses. Don't use one to store the other.
- If the use case calls for a relative time, do not manually construct or edit the date. Use your platforms date-creation/modification APIs, no matter how unnecessary they seem.
- Understand what is inside your platform's date types at rest. Understand which of your platform's date APIs pull in environmental information (time/tz/locale), as opposed to only using the arguments you pass it. Understand that your platform's 'print/stringify' function may be one of those aforementioned functions. Misunderstanding this often leads people to say inaccurate things. E.g. say your platform has a Date object that stores an epoch-based timestamp. People may say "the Date object is always in UTC", when really the Date object has no time offset, which is not the same thing.
- Understand that if you pass a date around platforms, it might accidentally be reserialized into the same absolute time, but a different relative time.
- Understand that there is a hierarchy of use cases, where each one has more complex requirements:
1. "Create/modify" timestamps; egg timers. (absolute time)
2. Alarm clocks (same clock time always).
3. One-time calendar events (has an explicit, static tz; same clock time if the user changes its day or time zone; different clock time if the user's time offset changes)
4. Recurring calendar events (same as above, except don't change the clock time if the user's time offset changed due to DST, as opposed to a geographic change)
5. Recurring calendar event with multiple participants (same as above, just remember that the attached tz is based on the creator, so the clock time will shift during DST for participants in a place without matching DST rules).
Note that a lot of platforms nowadays have built-in or 3rd party packages that automatically handle a lot of the rules in the above use cases.
Finally, understand that all those little weird things about dates (weird time zones, weird formatting conventions, legislative time zone changes, retroactive legislative time zone changes, leap days, leap seconds, times that don't exist), are good to know, but they will mostly be accounted for by the above understandings. You can get into them when you want to handle the real edge cases.