R1:2fc3acc9699f

Improve time localization code

Summary:
- We throw on a missing date right now, in the DateTime constructor. This can
happen in reasonable cases and this is display code, so handle it more
gracefully (see T520).
- This stuff is a little slow and we sometimes render many hundreds of dates
per page. I've been seeing it in profiles on and off. Memoize timezones to
improve performance.
- Some minor code duplication that would have become less-minor with the
constructor change, consolidate the logic.