How to add dates to fill in a date field

What I want is for Docuware to take the start date for a contract, add up the total months in lasts for, and then populate the field for the end date with the result. Any idea how to make this happen?

This can be done with Autoindex or Workflow by using the dateadd function 

Thanks, but I don't know how to use arithmetic functions. Where do I go to use them?