doi thoi gian PHP

<?php
$giodienra= "01:45:00";
$ngaydienra= "30/04/2014";
$timestamp1 = strtotime($giodienra);
$timestamp2 = strtotime($ngaydienra);
$timestamp = $timestamp1 + $timestamp2+7*3600;
echo "<br>timestamp: $timestamp";
//echo gmdate("M d Y H:i:s", 1396103400);
//$str = 'Mar 29 2014 14:30:00';
$start_time= "1398822300";
echo "<br><hr><br>";
echo "Test: ";
echo gmdate("m d Y H:i:s", $start_time);
?>