[php-src] Issue #12477: Subtracting 1 month from a DateTime or DateTimeImmutable

Issue: https://github.com/php/php-src/issues/12477 Author: sjoller ### Description The following code: ```php <?php $expirationDate = new DateTimeImmutable('2025-12-31 23:59:59'); echo $expirationDate->format('Y-m-d H:i:s') . chr(10); $newDate = $expirationDate->modify('-1 day'); echo $newDate->format('Y-m-d H:i:s') . chr(10); $newDate = $expirationDate->modify('-1 month'); echo $newDate->format('Y-m-d H:i:s') . chr(10); $newDate = $expirationDate->modify('-2 month'); echo $newDate->format('Y-m-d H:i:s') . chr(10); ``` [3v4l.org link](https://3v4l.org/utged) Resulted in this output: ``` 2025-12-31 23:59:59 2025-12-30 23:59:59 2025-12-01 23:59:59 <- Not a full month subtracted 2025-10-31 23:59:59 ``` But I expected this output instead: ``` 2025-12-31 23:59:59 2025-12-30 23:59:59 2025-11-30 23:59:59 <- What I need it to be 2025-10-31 23:59:59 ``` As shown, $expirationDate is set correctly and prints out as expected. Subtracting 1 day works as expected too, but then subtracting 1 month does *not* work - it subtracts 30 days, not the full 31 days of December month, landing me on the first day of December, not the last day of November. Finally subtracting 2 months again works as expected, subtracting the required 61 days, landing me squarely on the last day of October. Originally I used $expirationDate->sub(new DateInterval('P1M')) (using a DateTime object), but that did the exact same thing. I'm fairly sure it's not my setup acting wierd - testing on https://onlinephp.io/ yields the same results ### PHP Version PHP 8.2.11 (and PHP 7.4.x) ### Operating System Debian BookWorm (WSL)