Description:
------------
If substract 1 Month for 31/30/29 march => Not correctly subtract (possible problem to February Month).

Test script:
---------------
<?php

$intervalMonth = new DateInterval('P1M');

$dates = ['2019-01-30', '2019-02-28', '2019-03-29', '2019-03-30', '2019-03-31', '2019-04-30'];
foreach($dates as $date){
    
    $d = new DateTime($date);
    
    // Substract 1 Month
    $d->sub($intervalMonth);
    $subMonth = $d->format('Y-m-d');
    
    // Adding 1 Month
    $d->add($intervalMonth);
    $addMonth = $d->format('Y-m-d');
    
    // Echo information
    if(substr($date, 0, 7) === '2019-03'){
        echo '*';
    }
    echo 'Now: '.$date.'; Sub 1Month: '.$subMonth."; Add Month: ".$addMonth."\n";
}

echo "\n";
echo '* BUG';

Actual result:
--------------
Now: 2019-01-30; Sub 1Month: 2018-12-30; Add Month: 2019-01-30
Now: 2019-02-28; Sub 1Month: 2019-01-28; Add Month: 2019-02-28
*Now: 2019-03-29; Sub 1Month: 2019-03-01; Add Month: 2019-04-01
*Now: 2019-03-30; Sub 1Month: 2019-03-02; Add Month: 2019-04-02
*Now: 2019-03-31; Sub 1Month: 2019-03-03; Add Month: 2019-04-03
Now: 2019-04-30; Sub 1Month: 2019-03-30; Add Month: 2019-04-30

* BUG