Drop down list with time zone offset.

I saw on a web site http://aspfree.com/ when a user signed up they could set the user's time zone offset from a Drop Down List.  Has anyone done anything similar or have some code that does something similar.  We are having some time zone issues with our site and this would be tremendously helpful.  I did some mining on the net and can't find anything similar.  
 suggested better way, tutorials, articles, anything will be appreciated.
 BC  
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