Take care, if you have a DateTime Object on the 31h of January and add Da DateInterval of one Month, then you are in March instead of February.
For Example:
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// given the actual date is 2017-01-31
$today = new DateTime('now', $timeZoneObject);
$today->add(new DateInterval('P1M'));
echo $today->format('m');
// output: 03
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