Hmm... checkTimeZone64 reveals that glibc miscalculates time zones for posix timestamps above around 2^47. I've limited tests to values below that.

% env TZ=America/Los_Angeles date -d '5881580-7-1 00:00 UTC'
Mon Jun 30 05:00:00 PM PDT 5881580
% env TZ=America/Los_Angeles date -d '5881581-7-1 00:00 UTC'
Tue Jun 30 04:00:00 PM PST 5881581

> logBase 2 $ fromIntegral $ utcTimeToInt64 $ UTCTime (fromGregorian 5881581 7 1) 0
47.398743929757934

For reference, 1st July should always be in PDT according to the rule:

> (fmap renderPosixTz . _tzPosixTz) <$> loadTZFromDB "America/Los_Angeles"
Just "PST8PDT,M3.2.0,M11.1.0"

Thanks a lot!

I started working on this about a year ago, but for various reasons haven't finished. 😅 Do you mind if I cherry-pick from your code and merge it with whatever I have?

Also, I'm very busy this week, so I'll try to get to this sometime next week!

Awesome! I wasn't sure how to deal with localToPosix so uh... yeah go ahead :).

I just pushed test tweaks I was working on but I'll hold off with any more changes for now to give you time to glue things together.

... except I think it would be best not to depend on attoparsec so at the cost of additional approx 100 lines of code I've inlined what we need to parse POSIX-TZ. That's in case you don't already have something better of course ;)