Description:
------------
When subtracting an interval that contains months and days, the number of days subtracted is sometimes increased by an inconsistent amount.

Here's the 3v4l: https://3v4l.org/uIWq4

Test script:
---------------
$firstDayOfYear = new DateTimeImmutable('2021-01-01 00:00:00.000000');


for ($i = 1; $i <= 12; $i++) {
    $intervalOneEach = new DateInterval('P' . $i . 'M1D');
    echo $firstDayOfYear
        ->sub($intervalOneEach)
        ->diff($firstDayOfYear)
        ->format('%y years %m months %d days %h hours %i minutes %s seconds'), PHP_EOL;
}


Expected result:
----------------
0 years 1 months 1 days 0 hours 0 minutes 0 seconds
0 years 2 months 1 days 0 hours 0 minutes 0 seconds
0 years 3 months 1 days 0 hours 0 minutes 0 seconds
0 years 4 months 1 days 0 hours 0 minutes 0 seconds
0 years 5 months 1 days 0 hours 0 minutes 0 seconds
0 years 6 months 1 days 0 hours 0 minutes 0 seconds
0 years 7 months 1 days 0 hours 0 minutes 0 seconds
0 years 8 months 1 days 0 hours 0 minutes 0 seconds
0 years 9 months 1 days 0 hours 0 minutes 0 seconds
0 years 10 months 1 days 0 hours 0 minutes 0 seconds
0 years 11 months 1 days 0 hours 0 minutes 0 seconds
1 years 0 months 1 days 0 hours 0 minutes 0 seconds

Actual result:
--------------
0 years 1 months 2 days 0 hours 0 minutes 0 seconds
0 years 2 months 1 days 0 hours 0 minutes 0 seconds
0 years 3 months 2 days 0 hours 0 minutes 0 seconds
0 years 4 months 1 days 0 hours 0 minutes 0 seconds
0 years 5 months 1 days 0 hours 0 minutes 0 seconds
0 years 6 months 2 days 0 hours 0 minutes 0 seconds
0 years 7 months 1 days 0 hours 0 minutes 0 seconds
0 years 8 months 2 days 0 hours 0 minutes 0 seconds
0 years 9 months 1 days 0 hours 0 minutes 0 seconds
0 years 10 months 3 days 0 hours 0 minutes 0 seconds
0 years 11 months 1 days 0 hours 0 minutes 0 seconds
1 years 0 months 1 days 0 hours 0 minutes 0 seconds