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date_default_timezone_set('Europe/Moscow');
$date = strtotime("now");
$interval = 3;
$payment_periodicity_period = 'month';
$payment_date = strtotime('+'. $interval . ' ' . $payment_periodicity_period, $date);
$dtStr = date("c", $payment_date);
$payment_date_dt = new DateTime($dtStr);
$start_percents_dt = $payment_date_dt->modify('-'.$interval. ' '.$payment_periodicity_period);
//echo date('Y-m-d',$payment_date);
//echo "<br />";
//echo $start_percents_dt->format('Y-m-d');
$start_percents = date_timestamp_get($start_percents_dt);
//echo "<br />";
//echo $start_percents;
//echo "<br />";
$datediff = $payment_date - $start_percents;
$qty_month_period = floor($datediff/(60*60*24));
echo $qty_month_period;