hi,
you must have a start date right?? it is from start of the month or the start date is a fixed date ??
You can use FM HR_99S_DATE_PLUS_TIME_UNIT , it FM Add months(M)/days(D)/years(Y) to given date to get the future date.
refer the link which contains all the FM name about date process : Function Module related on Date calculations - ABAP Development - SCN Wiki