These requests for comments/change have lead to an implemented feature that has been successfully merged into the 3.1/Ascraeus branch. Everything listed in this forum will be available in phpBB 3.1.
This seems rather confusing and difficult to set in the UCP.
After selecting a timezone, I am given this massive unorganized list to sort through in order to find my local area. The only thing that selecting a timezone seems to do is to display an identifier in the huge list of areas showing where that timezone starts. What I think might be better is to use some javascript to hide all timezones outside of +-1 that was selected in the first place, that would make finding the correct one significantly easier.
The identifiers of all the sections might be actually better just kept in place all the time as well, so that people can see there are different sections sorted by timezone, then alphabetical (otherwise someone would just assume it is alphabetical, which makes finding something confusing).
Actually the list you see is only places that have exactly your current local time. With the current implementation after selecting a time you do not see any locations that have a different time than yours.