<!-- Collection of code snippets by Arne Vajhøj -->
<!-- posted to eksperten.dk, usenet and other places (2002-now) -->
<?php
if (date("d") <= 23) {
    $t1 = strtotime('first day of last month') + 23*24*60*60;
    $t2 = strtotime('first day of this month') + 22*24*60*60;
} else {
    $t1 = strtotime('first day of this month') + 23*24*60*60;
    $t2 = strtotime('first day of next month') + 22*24*60*60;
}
echo date('Y-m-d', $t1) . "\r\n";
echo date('Y-m-d', $t2) . "\r\n";
if ((new DateTime())->format("d") <= 23) {
    $t1 = (new DateTime('first day of last month'))->add(new DateInterval('P23D'))->getTimeStamp();
    $t2 = (new DateTime('first day of this month'))->add(new DateInterval('P22D'))->getTimeStamp();
} else {
    $t1 = (new DateTime('first day of this month'))->add(new DateInterval('P23D'))->getTimeStamp();
    $t2 = (new DateTime('first day of next month'))->add(new DateInterval('P22D'))->getTimeStamp();
}
echo date('Y-m-d', $t1) . "\r\n";
echo date('Y-m-d', $t2) . "\r\n";
?>