This is a function i used in the past for substracting a date from another one related with your question, my principe was to get how many days, hours minutes and seconds has left until a product has expired :
$expirationDate = strtotime("2015-01-12 20:08:23");
$toDay = strtotime(date('Y-m-d H:i:s'));
$difference = abs($toDay - $expirationDate);
$days = floor($difference / 86400);
$hours = floor(($difference - $days * 86400) / 3600);
$minutes = floor(($difference - $days * 86400 - $hours * 3600) / 60);
$seconds = floor($difference - $days * 86400 - $hours * 3600 - $minutes * 60);
echo "{$days} days {$hours} hours {$minutes} minutes {$seconds} seconds";