- date_diff: documentation ( source)
<?php
$sStartDate = DateTime::createFromFormat('Y-m-d H:i:s', '2013-05-13 21:56:00');
$sEndDate = DateTime::createFromFormat('Y-m-d H:i:s', '2013-05-14 21:56:01');
$diff = date_diff($sStartDate, $sEndDate, true);
$days = $diff->format('%a');
if (1 <= $days) {
echo '24 hours or more.';
} else {
echo 'less than 24 hours.';
}