Description:
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$total_seconds = strtotime("2015-04-00 00:00:00") - strtotime("2015-03-00 00:00:00");

This code shows that $total_seconds = 2674800 (This is False)
True answer is 2678400 ($total_seconds + 3600)
And the Same problem is in another month 

$total_seconds2 = strtotime("2015-11-00 00:00:00") - strtotime("2015-10-00 00:00:00");

This code shows $total_seconds2 = 2682000 (This is False)
True answer is 2678400 ($total_seconds - 3600)

This problem is in between March-April && Sep-Oct in Many years. Like 2011,2012,2013,2014 etc.

Test script:
---------------
<?php

$total_seconds = strtotime("2015-04-00 00:00:00") - strtotime("2015-03-00 00:00:00");

echo $total_seconds;
echo "<br>";

$total_seconds2 = strtotime("2015-11-00 00:00:00") - strtotime("2015-10-00 00:00:00");

echo $total_seconds2;
?>