I’ve been on four projects in the last year that all had issues with Timezones, user locales, UTC, and everything else in between that relates to time and date calculations.
Timezones are complex, but rarely do we deal with them at the beginning. Instead we deal with them when the first issue rolls in and then we have patch or hack the implementation because to fix them in the source would require cataclysmic changes that would affect all users.
Maybe it’s not timezones for you, maybe it’s something else. Whatever it is, if it’s going to affect all of your customers if you ever have to change it later on. It’s probably something you want to put some effort into today.