- strtotime: documentation ( source)
<?php
$device["time_stamp"] = '2014-03-24T19:13:45.000Z';
$device_time_stamp = strtotime($device["time_stamp"]);
$last_update_threshold_time_stamp = strtotime('-10 minutes');
$result = ($device_time_stamp <= $last_update_threshold_time_stamp ? true : false);
echo $result;