Here's one way from modules I just happened to have lying around on my system (you'll probably need to get Date::Manip from CPAN though)

#!/usr/bin/perl -l

use Date::Manip;

$date = "Wed Feb 18 2004, 11:03:45 AM EST";
print UnixDate(ParseDate($date),"%s");
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I seem to recall that Date::Calc also has a routine to convert from almost any date format to "unix time"