Creates a copy of this and adds the specified number of months to the copy.
Add negative values to subtract months.
The day of the month of the resulting DateTime is clamped to the number of days in the updated calendar month. For example, if adding 1 month to 31st January 2018, the result would be 28th February 2018. In 2020 (a leap year), the result would be 29th February.
the number of months
the newly created DateTime which should be freed with