Time zones are part of it, but also daylight savings is a real pain in the ass. And like you said it gets particularly complicated when you’re dealing with a system that deals with these things as an afterthought, which seems to be a lot of older libraries for time. For instance, the Java date utils are a nightmare and are now considered semi deprecated replaced by a new java.time api. That is, of course, no help for the ridiculous amount of things that depend on these stupid date utils and no one wants to spend the dev hours to refactor.