use Time::Local;

$start = '2004-11-15T18:59:52.863Z';
$end   = '2004-11-15T19:09:52.972Z';

@parts = $start =~ /^(.{4})-(.{2})-(.{2})T(.{2}):(.{2}):(.{2})\.(.{3})
+Z$/;
$start_secs = pop(@parts) / 1000;
$start_secs += timegm reverse @parts;

@parts = $end =~ /^(.{4})-(.{2})-(.{2})T(.{2}):(.{2}):(.{2})\.(.{3})Z$
+/;
$end_secs = pop(@parts) / 1000;
$end_secs += timegm reverse @parts;

$diff = $end_secs - $start_secs;

printf("%.3f$/", $diff);  # 600.109
[download]