Time Zone by latitude and longitude

Hi,

I want to inlcude in my small software a module that displays time zone
info from given coordinates. I want to have same result as from
http://www.earthtools.org/timezone-1.1/40.71417/-74.00639 (example from
New York). This web service works fine, but I want to have my own
module which computes time zone from any digital data.

Do You know any source which I can include to my software that will
suit my demands?
I know about Olson tz database, but it returns timezone according to
the largest town in the area (I'm not 100% sure, but i think that it
works this way), but I want coordinates as input

thanks for any response