Calculate recurring billing dates

RecurringBillingDate.php

<?php

// Create a new DateTime object with the first billing date.
$lastDate = new DateTime('2019-01-31');

// Clone the date object or the original date will be altered.
$nextDate = clone $lastDate;
$periodUnit = 'M';

// Add the number of months.
$numberMonths = 1;
$nextDate->add(new \DateInterval('P' . $numberMonths . $periodUnit));

// Check the months amount and correct if it is needed.
if (intval($nextDate->format('n')) > (intval($lastDate->format('n')) + $numberMonths) % 12 &&
    (intval($lastDate->format('n')) + $numberMonths) != 12) {

  $nextDate->modify('last day of -1 month');
}

echo $nextDate->format('Y-m-d');