- date: documentation ( source)
- strtotime: documentation ( source)
<?php
$endDate = strtotime('2015-03-30 12:00:01');
$endDate -= $endDate % 1800;
$endDate = date('Y-m-d H:i:s', $endDate);
$startDate = date('Y-m-d H:i:s', strtotime($endDate . " - 1800 seconds"));
echo $startDate . ' - ' . $endDate;