[shadowgovt]

Time calculation is deceptively hard.

Here's a fun one: are timezones a mathematical construct or a physical construct? By which I mean: is the core of the problem to get the math right or to account for the geography of the planet?

It's a trick question. Timezones are a political construct. If your software is using timezones, it needs to somehow(1) account for changes to law, internationally. Individual countries can choose, for example, to use or stop using daylight savings time. Some have even chosen to shift the longitudinal timezone they consider themselves to be in.

(1) practically speaking, relying on tzdata (https://en.wikipedia.org/wiki/Tz_database) is a great solution to this problem for most use cases... But you need to keep your copy of it updated, because laws change.