Занятно, но баг остался аж с марта 2011 года.
<?php
date_default_timezone_set("Europe/Moscow");
$today = date("d-m-Y",mktime(0,0,0,4,28,2013)); // 28-04-2013, воскресенье
$lastWeek = strtotime("last week $today");
$thisWeek = strtotime("this week $today");
$nextWeek = strtotime("next week $today");
echo "last week - ".$lastWeek." / ".date("d-m-Y H:i\n",$lastWeek);
echo "this week - ".$thisWeek." / ".date("d-m-Y H:i\n",$thisWeek);
echo "next week - ".$nextWeek." / ".date("d-m-Y H:i\n",$nextWeek);
результат работы:
last week - 1366574400 / 22-04-2013 00:00
this week - 1367179200 / 29-04-2013 00:00
next week - 1367784000 / 06-05-2013 00:00