Best way to add months (duration) to a date formula?

Has anyone been able to establish the most elegant way of adding the duration in months to a date formula?

I know it's not straightforward because there are lots of variables in days in the month, and leap years etc.  However, it would be great if someone has gone through this and was prepared to share their working?

[Date Agrmnt Signed] + [Agreement Duration (Months)]

Thanks.