Thanks for the PR! LGTM, I just have one question about the rounding. I guess it might be an arbitrary choice, but why do we want to round up? Do any other systems implement avg for temporals and also round up instead of down? Maybe MySQL?

I just added rounding because we always do that for fixed point division/casting. It's only ±1µs so not a big deal either way.

I can't find any other system that does this (Oracle, MS-SQL, ...) - although there are lots of people asking and being given workarounds that involve casting to a numeric value. As the PR shows, it's not difficult, but no one seems to do it.

(Funny factoid about MySQL dates: Because they support zeros for day and month fields to mean "all month" and "all year", their timestamps are not dense numbers, so there is no way to average their date values!)