- date_default_timezone_set: documentation ( source)
- strtotime: documentation ( source)
<?php
date_default_timezone_set("Europe/Brussels");
$test = 1537430400; //20-09-2018 10:00
$test2 = 1537438500; //20-09-2018 12:15
echo strtotime("next monday", $test); // expected 24-09-2018 10:00
echo " -- ";
echo strtotime("next monday", $test2); // expected 24-09-2018 12:15