sub my_fmt_duration {
	my @t = shift->in_units('hours','minutes','seconds','nanoseconds');
	$t[3]/=1000000; # ns->ms
	return sprintf("%02d:%02d:%02d:%03d",@t);
}

print my_fmt_duration($first),"\n";
print my_fmt_duration($last ),"\n";
$last->subtract_duration($first);
print my_fmt_duration($last ),"\n";