I tried the same formula which is given in https://developer.salesforce.com/forums/ForumsMain?id=906F00000008vsfIAA with my fields substituted(added 36 months in my case) and I got an error saying Mod expected Number but received Date.Here it is:
DATE (
YEAR(Commencement_Date__c) + FLOOR((MONTH(Commencement_Date__c) + 36 - 1)/12),
/*MONTH*/
CASE(MOD(MONTH(Commencement_Date__c) + 36, 12 ), 0, 12, MOD(MONTH(Commencement_Date__c)+ Commencement_Date__c, 12 )),Commencement_Date__c,Commencement_Date__c)/*DAY*/
MIN(DAY(Commencement_Date__c),
CASE(MOD(MONTH(Commencement_Date__c) + 36,12), 9, 30, 4, 30, 6, 30, 11, 30, 2, /* return max days for February dependent on if end date is leap year */
IF(MOD(YEAR(Commencement_Date__c) + FLOOR((MONTH(Commencement_Date__c) + 36)/12), 400) = 0 || (MOD(YEAR(Commencement_Date__c) + FLOOR((MONTH(Commencement_Date__c) + 36)/12), 4) = 0 && MOD(YEAR(Commencement_Date__c) + FLOOR((MONTH(Commencement_Date__c) + 36)/12), 100) <> 0 ), 29,28), 31)) )
Please help me to resolve this.
Thanks.