guessing timezone based on country/state/city

I have an event CMS where really, I should know the TZ of the events, which can happen world wide. However the user base is not computer heads, so I can't just "ask them". So, I'm faced with the prospect of making an educated guess based on the Country, State and Capital City information provided by the user... possibly I could interface with the files in /usr/share/zoneinfo/, or perhaps there's a database I can download to hook into. If I was limiting myself to just Australia, it'd be quite easy to hard code it, but since I'm targeting clients and events world-wide, I really need something less "manual". TIA, Justin French