Is it getting-the-data which isn't trivial? You can write a crawler for wikipedia. Each country has their time zones in their entry (nicely formatted which ma

Is it getting-the-data which isn't trivial? You can write a crawler for wikipedia. Each country has their time zones in their entry (nicely formatted which makes it easy for parser to find), with daylight saving information too. Many countries, are, of course spanned across a few time zones, but that shouldn't matter. The root page for your crawler could be: http://en.wikipedia.org/wiki/List_of_countries_by_population http://en.wikipedia.org/wiki/List_of_countries_by_population, for instance.