A website called EarthTools has a web service for that.
http://www.earthtools.org/webservices.htm#timezone
You can pass in latitude and longitude like so:
h ttp://www.earthtools.org/timezone/40.71417/-74.00639
And it spits out the time zone and a bunch of other info:
<?xml version="1.0" encoding="ISO-8859-1" ?>
<timezone xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="http://www.earthtools.org/timezone.xsd">
<version>1.1</version>
<location>
<latitude>40.71417</latitude>
<longitude>-74.00639</longitude>
</location>
<offset>-5</offset>
<suffix>R</suffix>
<localtime>4 Dec 2005 12:06:56</localtime>
<isotime>2005-12-04 12:06:56 -0500</isotime>
<utctime>2005-12-04 17:06:56</utctime>
<dst>False</dst>
</timezone>
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