One thing I have learned from HN is that datetime issues are hard, prolific, programming language agnostic, and not to trust myself to get the logic right. (The same applies to floats.)
Another thing programmers should know and fix is that most of it is self-inflicted. When there’s no easy way to add a day, people add 86400000 and stumble upon a leap second. When time is not needed, they use fixed hours and fail at timezones. And so on. Most date libraries provide mostly trivial and at the same time low-level use-cases, so people do all the stupid math with what essentially is an irregularly-based number.
