I have not counted the number of timezones in the dropdown but assuming 24 timezone offsets that comes out to 20 timezones per offset, which is about one page of a dropdown box (looking at the forum list on area51 right now in firefox 3). Assuming you know what your timezone offset is you would need to look through at most two such pages of timezones.
The code should be here: https://github.com/p/phpbb3/compare/dev ... z-handling
Edit: looks like the last commit broke migrations, which I forgot about. https://github.com/phpbb/phpbb3/pull/175 needs to be merged for me to work on fixing timezone branch.
Getting this error on login:
[phpBB Debug] PHP Notice: in file /includes/functions.php on line 4527: Undefined index: tz
Could be irrelevant.
You can see the timezone selector by pressing the agreement button on this page: http://108.35.7.206:8071/boards/tz1/ucp ... e=register


