I'm trying to calculate the future date when the contract ends. I have 2 custom fields:

Service Effective Date which is a picklist showing 1, 2 and 3 years respectivelyand Contract Activation Date which contains date when contract started and activated.
If only one year then End date of contract year should be 12 months from Service_Effective_Date__c
If two years then End date of contract year should be 24 months from Service_Effective_Date__c
If three years then End date of contract year should be 36 months from Service_Effective_Date__c