Count forward from date

Hi all,

I have a date field, lets say it's 01/01/2015.  I have a separate field containing a number of months, lets say 6 months.

So "Contract start date " = 01/01/2015

"Length of contract" = "6"

"Contract end date" = ????

That is my question.  How do I count forward (the number of months) from that start date, and auto populate the ends date with the correct date??

Many thanks