I once developed an electronic program guide for a cable TV company in New Zealand and I’d lose my mind if I had to use timezones. The basic rule of thumb was:
a) Internally you use UTC religiously. UTC is the same everywhere on Earth, time always goes forward, most languages have classes that represent instants, durations etc. In addition you make damned sure your server time is correct and UTC.
b) You only deal with timezones when presenting something to a user or taking input from a user
Prior to that I had worked for a US trading company that set all their servers to EST and was receiving trades through the system which expressed time & date ambiguously. Just had to assume everywhere that EST was the default but it was just dumb programming and I bet to this day every piece of code they develop has time bugs.