Timezones for multi-site installations

Greetings,

Is there a module that implements setting of timezones?

Yes, I know you can set it via the putenv(TZ='<zone>') in config.php, but is there a module that enables users to set their own timezones in a mult-site setup?

I need a way to provide users this capability preferrably with a simple UI rather than directly accessing and writing on such files like config.php.

If there's a module that does this, please point me to it.. or tell me if I have to come up with my own..