The details I never really thought about until I encountered timezone issues was:

There are points in time that don't exist, and there are points in time that are ambiguous.

If we adjust the time forward by 1 hour, then then 2:30am doesn't exist. If we adjust time backward, then 2:30am is ambiguous.

2am-3am is only safe for certain countries.

America/Santiago (Chile), for example, adjusts their daylight saving on midnight. On a certain day, midnight doesn't exist because it immediately jumps to 1am. If you are building an analytics chart by day, you will encounter this issue. I wrote about several weird time zone edgecases here: https://tanin.nanakorn.com/edgecases-for-timezones/

To add more confusion, IIRC Java and Ruby handle these cases differently where Ruby raises exceptions (e.g. InvalidTime and AmbiguousTime) but Java doesn't.

I caused 3 incidents in a row at Stripe in my first month because of this. I thought I was gonna be fired. Luckily I wasn't...