Subtracting 1 month from a DateTime or DateTimeImmutable

[sjoller]

Description

The following code:

<?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

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)

[KapitanOczywisty]

Duplicate of #9454

[damianwadley]

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.

It is subtracting one month. But it's doing so literally: 2025-12-31 - 1 month = 2025-11-31.
Problem is that's not a real date, so PHP chooses to return to you the most reasonable date it can: by overflowing the date to 12-01.

You need to decide what you want to happen in this situation, then implement code to enforce that behavior; assuming you're okay with it, not just the advantages but the disadvantages too, the "last day of previous month" mentioned in 9454 is a simple way of handling that particular calculation.

[sjoller]

Thanks for explaining the inner workings - I just came to sort of the same conclusion, from example 2 on the modify() documentation page.
As I not only needed to get the last day of previous month, I needed the last day of the penultimate month too, but luckily a string like last day of -2 month will happyly parse into a DateTime/DateTimeImmutable.
Actually last day of previous months previous month worked as well, but the cringe.is strong in that one.