How do I add days to a date?

Not sure where to ask this, but I want to add days to a date. So, for example, the users puts in a target date when they want the item to arrive. Now, depending on a few different things, the actual ship date may be a few days different.

What I did was create a target date input field and a ship date formula field. In the formula field I tried to do target_date = ship_date + 5d

What am I missing here?

Thanks