There are a slew of modules available to do date calculations. The Swiss army chainsaw date module is Date::Manip. Date::Calc however provides Add_Delta_YM which "can be used to add a year and/or month offset to a given date" and "this function does no ``wrapping'' into the next month ... it simply truncates the day to the last possible day of the resulting month".

Store the start date and add n months to get the next update date to avoid the truncation issue.