Add A Number Of Months To Date

I have a simple query which returns a purchase date plus the number of months warranty an item has. What I need to do is add these months to the purchase date to get a warranty expiry date. What is the best way to do this? My query and results are below, I need an extra column in the results with that calculated warranty date:

select item, purchase_date, warranty_period
from inventory where item like '05MC%'

item purchase_date warranty_period
-------------------- ----------------------- ---------------
05MC0001 2014-07-22 00:00:00 36
05MC0002 2014-07-11 00:00:00 36