Timezone lat/lon boundaries?

[WhoIsJohnGalt?]

Do you know of a source, commercial or otherwise, or lat/lon 
boundaries of timezones?  I have an application that would be
considerably improved if, given the lat/lon of a size, the
timezone and DST calculations could be made.

If it matters, I currently only need this for the US, but it
would be nice to include some world timezones as well.

Thanks for your help.  Please email replies to dhagberg at gds.com.

			-=- D. J. Hagberg