Take a look at
DateTime its only a developer release at the moment, but its moving ahead fast. I imagine the people at the
DateTime project would like to hear how it goes for you (email
datetime@perl.org).
DateTime aims to be the difinitive perl module for working with dates and times. The DateTime::Timezone module(s) are based on the OlsenDB - the central source of information on timezones and daylight savings so they should be right. There is talk on the list about how to keep these files up to date - which will be sorted out by the time you'd need to update them :)
Try using DateTime something like this:
my $realtimehere = DateTime->now;
my $realtimethere = DateTime->new(time_zone => 'America/Los_Angeles');
my $difference = $realtimehere - $realtimethere;
The PODumentation will explain how to use the $difference duration. Also note that you might have to feed a time into the new() method, but I'm sure someone else will chime in and tell you.
The code is completely untested because I can't install DateTime on my mac here at the moment. Of course, read the documentation and then test it :)