https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...
E.G. in a conversation "Lets open the shop at 9am every day that it isn't closed." Is a fairly simple recurrence, with some exceptions*. If timezones change the scheduled time remains evaluated again on each day.
The BCL-provided DateTime was really confusing, especially when you just needed a Date. They eventually got around to including a DateOnly, but before that happened I switched to a library called "Noda" (or Joda in Java) and after a bit of a learning curve, it made everything a lot easier to reason about.
It has LocalDates and LocalDateTimes, as well as Instants to store UTC times. It also offers ZonedDateTimes, but I don't use those as much. I work in healthcare. And so many regulations involve strictly dates. Like, "You have 5 days to do X", not "You have 120 hours to do X", and as such, storing the time with a lot of this data can add more complexity.
The calculations themselves are usually pretty easy with a few exceptions, it's just that there are TONS of them. And they all slightly depend on each other. And they change. Often.
After having a bunch of problems with dealing with Dates coded as DateTime, I've begun coding dates as a Date primitive, and wrote functions for calculation between dates ensuring that timezone never creeps its way into it. If there is ever a DateTime string in a Date column in the database, it's impossible to know what the date was supposed to be unless you know you normalized it at some point on the way up.
Then I found that a lot of DatePicker libraries, despite being in "DATE" picker mode, will still append a local timezone to its value. So I had to write a sanitizer for stripping out the TZ before sending up to the server.
That said, I am pretty excited about Temporal, it'll still make other things easier.