[growse]

This doesn't work reliably for future timestamps. If a user submits a local timestamp and you then convert it to UTC, you're using the current definition of what we think that offset will be at that future time.

However, timezone definitions change. By doing the conversion, you've got to remember to check that the offset between what you stored and the user's local hasn't changed since the time you did the conversion. How often and when you do this checking is ... not obvious.

[out_of_protocol]

UTC or unix timestamp gets you moment of time. You know exactly moment event happened. Definitions can change, but usually not retroactively. TZdata is a database of such changes, and stores definition changes, basically all *nix OSes depend on it to cast moment of time into timezone. You can always convert timestamp in the past into desired timezone. If you work with calendars and want you know what "04:00 tomorrow" means - good luck, you'll need it. as example, "04:00 tomorrow" may not exist at all

[out_of_protocol]

* ignoring leap seconds