To capture the expiration date of warranty based on installation date and warranty length (1 year by default), the administrator should use a formula as the default value of Warranty Expiration Date field on Installation object. The formula can calculate one year from installation date using DATE function or DATEVALUE function. For example, DATE(YEAR(Installation_Date__c) + 1 , MONTH(Installation_Date__c) , DAY(Installation_Date__c)) will return one year from installation date. Creating a formula field, adding a validation rule, or including Warranty Expiration Date field on mobile page layout will not capture expiration date based on installation date and warranty length. Reference: https://help.salesforce.com/s/articleView?id=sf.formula_using_date_datetime.htm&type=5