- mktime: documentation ( source)
- date_default_timezone_set: documentation ( source)
- date: documentation ( source)
- strtotime: documentation ( source)
<?php
date_default_timezone_set("America/Chicago");
$timestamp = mktime(0,0,0,11,1,2014);
$a = strtotime('4 thursdays', $timestamp);
$b = strtotime('first thursday + 3 weeks', $timestamp);
echo date(DateTime::ISO8601, $a), "\n";
echo date(DateTime::ISO8601, $b), "\n";