Description:
------------
When I'm adding (for example) months to the datetime, it shouldn't matter if I add months one-by-one by calling the "modify" method multiple times, or at once with one call.

I know it's caused by the February and I don't care if "1 month" means 30, 31, ... days, but I think that there should be a consistent result regardless the way the "modify" method is used.

Test script:
---------------
$today = new \DateTimeImmutable('2020-01-30 11:00:00');

var_dump($today->modify('+2 months')->format('Y-m-d H:i:s'));
var_dump($today->modify('+1 month')->modify('+1 month')->format('Y-m-d H:i:s'));

Expected result:
----------------
string(19) "2020-03-30 11:00:00"
string(19) "2020-03-30 11:00:00"

or

string(19) "2020-04-01 11:00:00"
string(19) "2020-04-01 11:00:00"

Actual result:
--------------
string(19) "2020-03-30 11:00:00"
string(19) "2020-04-01 11:00:00"