- date_diff: documentation ( source)
<?php
$sStartDate = DateTime::createFromFormat('2013-13-05 21:56:00');
$sEndDate = DateTime::createFromFormat('2013-15-05 21:56:01');
$diff = date_diff($sStartDate, $sEndDate, true);
$days = $diff->format('%a');
echo $days;
if (1 <= $days) {
echo '24 hours or more.';
} else {
echo 'less than 24 hours.';
}