#!/usr/bin/perl 
use strict; 
use warnings; 
 
use DateTime; 
use DateTime::TimeZone; 
 
my @dates = qw(20100815 20101120 20110217 20110320); 
my $time_zone = 'America/Montreal'; 
 
my %tz_opts = (time_zone => $time_zone); 
my $tz = DateTime::TimeZone->new(name=>$time_zone); 
foreach my $date (@dates) { 
    @tz_opts{'year','month','day'} = $date =~ m/(\d{4})(\d\d)(\d\d)/; 
    my $dt = DateTime->new(%tz_opts); 
    my $offset = $tz->offset_for_datetime($dt); 
    print "$date offset is $offset\n"; 
} 
 
Output: 
20100815 offset is -14400 
20101120 offset is -18000 
20110217 offset is -18000 
20110320 offset is -14400 
 
Works with 30 minute timezones as well: 
 
Time Zone: America/St_Johns 
20100815 offset is -9000 
20101120 offset is -12600 
20110217 offset is -12600 
20110320 offset is -9000 
 
Time Zone: America/Halifax 
20100815 offset is -10800 
20101120 offset is -14400 
20110217 offset is -14400 
20110320 offset is -10800

use strict; 
use warnings; 
 
use Date::Calc qw(Date_to_Time Timezone); 
my @dates = qw(20100815 20101120 20110217 20110320); 
 
foreach my $date (@dates) { 
    my ($year,$month,$day) = $date =~ m/(\d{4})(\d\d)(\d\d)/; 
    my $time = Date_to_Time($year,$month,$day,0,0,0); 
    my ($D_y,$D_m,$D_d, $Dh,$Dm,$Ds, $dst) = Timezone($time); 
    my $offset = $Dh * 3600 + $Dm * 60 + $Ds; 
    print "$date offset is $offset Daylight Savings: $dst\n"; 
}     
 
Output: 
20100815 offset is -14400 Daylight Savings: 1 
20101120 offset is -18000 Daylight Savings: 0 
20110217 offset is -18000 Daylight Savings: 0 
20110320 offset is -14400 Daylight Savings: 1