Instead of trying to parse dates by hand, which can get to be really tedious (believe me, I've tried), a better idea is to make use of one of the date-parsing modules available on CPAN. I regularly use Date::Parse:

#!/usr/bin/perl -w

use Date::Parse;

my $time = str2time("Mon Apr 18 15:17:29 2005 EDT");

print "That date is ", localtime($time). " as PDT.\n";
print "That date is ", gmtime($time).    " as GMT.\n";

__END__
That date is Mon Apr 18 12:17:29 2005 as PDT.
That date is Mon Apr 18 19:17:29 2005 as GMT.
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Another alternative is Date::Manip, which I haven't used but is fairly popular (AFAIK). There is also the DateTime Project. Unless performance is critical and you know your data well, you should probably stick to one of these solutions, and not reinvent the date-parsing wheel.