Calculate the next month pay date

function nextBillDate($firstBillDate) {
	$firstBillDateVal = strtotime($firstBillDate);
	$todayVal = strtotime(date('Y-m-d', strtotime("+8 hours")));

	if ( $todayVal < $firstBillDateVal) {
		$nextBillDateVal = $firstBillDateVal;  // first bill date in future 
	} elseif ($todayVal == $firstBillDateVal) {
		$nextBillDateVal = strtotime('+1 month -1 day', $firstBillDateVal);  // today is first bill date
	} elseif ($todayVal > $firstBillDateVal) {
		$this_year_date = sprintf("%s-%s",  date('Y-m', strtotime("+8 hours")), date('d', $firstBillDateVal));
		$pay_date = date('Y-m-d', date(strtotime('-1 day', strtotime($this_year_date))));
		$nextBillDateVal = strtotime($pay_date);
		if ($nextBillDateVal < $todayVal) {
			$nextBillDateVal = strtotime("+1 month", $nextBillDateVal);
		}
		$nextBillDateVal = $nextBillDateVal;
	}
	return(date('Y-m-d', $nextBillDateVal));
}

printf("Next Bill Date: %s", nextBillDate("2019-02-29"'));