And for the event in the past, respectively, you need datetime and the then-effective offset, not timezone, if you want to know the "what was the local time on

And for the event in the past, respectively, you need datetime and the then-effective offset, not timezone, if you want to know the "what was the local time on the actual physical wall clock?". Unless you're sure the tzdata were correct when the timestamp was taken, and your current tzdata have correct historical info, you can't just take UTC timestamp+timezone and convert it to local time. And I've seen enough machines that had deliberately wrong timezones for some time.