I’m a .NET dev, I don’t have a concept of “just use a library.” Everything is a library. I don’t mean “using int for datetimes is ok as long as you label it utc,” I just mean “don’t deal with time zones.”

Unix is the easiest format I’ve used. It’s easy to parse, it’s consistent, there’s not usually competing unix like formats, it converts perfectly to other time formats, most file explorers can immediately sort it correctly, and it’s clearly the date from which the universe spawned into existence.