Salesforce Formula:

DATE(
    YEAR(Contract_Start_Date__c) + FLOOR((MONTH(Contract_Start_Date__c) + Contract_Term__c - 1) / 12), 
    MOD(MONTH(Contract_Start_Date__c) + Contract_Term__c - 1, 12) + 1, 
    DAY(Contract_Start_Date__c)
) -1