#!/usr/bin/env perl -l

use strict;
use warnings;

use Time::Piece;

my $start = '2013-10-28T18:59:52.863Z';
my ($whole, $ms) = $start =~ /([^.]+)(.*)Z$/;
my $t1 = Time::Piece->strptime($whole => '%Y-%m-%dT%H:%M:%S');

my $t2 = gmtime;

my $diff = $t2 - $t1;

print 'Whole seconds diff: ', $diff;
print 'Fractional seconds diff: ', $diff - $ms;

##</code><code>##

Whole seconds diff: 46979
Fractional seconds diff: 46978.137

##</code><code>##

$ date -u
Tue 29 Oct 2013 08:02:51 UTC

##</code><code>##

$ perl -E 'say(5*60*60 + 8 + 8*60*60 + 2*60 + 51)'
46979