Доброе утро.
Вот Вам меленький пример, как посчитать разницу между датами.
$first = strtotime('04-02-2017 06:30');
$second = strtotime('14-03-2017 10:30');
$start = new DateTime(date('Y-m-d H:i', $first), new DateTimeZone('Europe/Moscow'));
$end = new DateTime(date('Y-m-d H:i', $second), new DateTimeZone('Europe/Moscow'));
$diff = $end->diff($start);
printf("The two dates have %d weeks, %s days, " .
"%d hours, %d minutes, and %d seconds " .
"elapsed between them.",
floor($diff->format('%a') / 7),
$diff->format('%a') % 7,
$diff->format('%h'),
$diff->format('%i'),
$diff->format('%s'));
