$today = (new DateTimeImmutable())->setDate(2023, 10, 31)->setTime(0, 0, 0);
$prevMonth = $today->sub(new DateInterval('P1M')); // subtract 1 month
$nextMonth = $today->add(new DateInterval('P1M')); // add 1 month
// prev month: 2023-10-01
// today: 2023-10-31
// next month: 2023-12-01
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2023.10.19 17:04