public function test() {
$lastMonthLastDay = date("Y-m-d", strtotime("last day of -1 month"));
echo $lastMonthLastDay . '<br/>';
$month = '2020-04';
$lastMonthLastDay = date("Y-m-d", strtotime("first day of {$month}"));
echo $lastMonthLastDay . '<br/>';
$month = '2020-01';
$lastMonthLastDay = date("Y-m-d", strtotime("{$month} +1 month -1 day"));
echo $lastMonthLastDay . '<br/>';
$month = '2020-03';
$lastMonthLastDay = date("Y-m-d", strtotime("{$month} first day of -1 month"));
echo $lastMonthLastDay . '<br/>';
$month = '2020-04';
$lastMonthLastDay = date("Y-m-d", strtotime("{$month} last day of -1 month"));
echo $lastMonthLastDay . '<br/>';
$month = '2020-03';
$lastMonthLastDay = date("Y-m-d", strtotime("{$month} first day of +1 month"));
echo $lastMonthLastDay . '<br/>';
$month = '2020-04';
$lastMonthLastDay = date("Y-m-d", strtotime("{$month} last day of +1 month"));
echo $lastMonthLastDay;
}