<?php
// Set timezone to something that has DST
date_default_timezone_set('Europe/Helsinki');
// 2010-03-29 is the first day in DST
echo date("Y-m-d H:i\n", mktime(0, 0, 0, 3, 29, 2010) - 86400);
echo date("Y-m-d H:i", strtotime('-1 day', mktime(0, 0, 0, 3, 29, 2010)));