I tried to have a look at an example that was posted but I'm really sturggling with it and hope I can get some community assistance. 

I have a "Service Start Date" field in Deals and I also have a "Number of Months" which is simply how many months the contract goes for. 

Based on the Service Start Date + Number of Months, I'd like to get a "Expiry Date" output.

I tried to go off this link but wasn't able to make it work.