DATE(
    YEAR(TODAY()),
    MONTH(TODAY()),
    CASE( MONTH(TODAY()),
        1, 31,
        2, IF( MOD( YEAR(TODAY()), 4) = 0, 29, 28),
        3, 31,
        4, 30,
        5, 31,
        6, 30,
        7, 31,
        8, 31,
        9, 30,
        10, 31,
        11, 30,
        12, 31,
        DAY(TODAY())
    )
)

for(My_Object__c o : trigger.new){
    o.Contract_End_Date__c = o.Contract_Start_Date__c.addMonths(o.Contract_Length__c);
}