<?php
// your code goes here
$date1 = new DateTime('2017-12-13 13:06:21.602');
$date2 = new DateTime('2017-12-13 13:06:18.102');
$interval = $date1->diff($date2);
echo $interval->s+$interval->f;
//
PD9waHAKCiAgICAvLyB5b3VyIGNvZGUgZ29lcyBoZXJlCiAgICAkZGF0ZTEgPSBuZXcgRGF0ZVRpbWUoJzIwMTctMTItMTMgMTM6MDY6MjEuNjAyJyk7CiAgICAkZGF0ZTIgPSBuZXcgRGF0ZVRpbWUoJzIwMTctMTItMTMgMTM6MDY6MTguMTAyJyk7CiAgICAkaW50ZXJ2YWwgPSAkZGF0ZTEtPmRpZmYoJGRhdGUyKTsKICAgIGVjaG8gJGludGVydmFsLT5zKyRpbnRlcnZhbC0+ZjsKICAgIAogICAgLy8KICAgIHZhcl9kdW1wKCRpbnRlcnZhbCk7CgoK