I was looking to do something similar and from another post in the community ("Calculating a new date +/- X days"), I finally got it to work for me with the following coding (after deleting the calculation field and creating a new one, another tip from that post):
var zover = @Start Date
zover.setDate(@Start Date.getDate() + @Duration);
I created a variable (zover) to hold the original date (@start date) and then replaced the original date with the calculated date in zover. @duration was a field I created in the template where I entered the amount of days I wanted to add. By the way, I tested the code with a specific number instead of @duration and that worked, too.
the getDate() function apparently returns the day of the month and the setdate() function sets the day of the month of whatever month the object is already holding, in my case it was the start date's month that I placed into the over variable zover.
There are probably a number of different ways to approach this issue, so if you find another way in your search please post it back here.
Full disclosure: I am not a programmer so I am not sure my explanation is correct and, in any case, I would not be able to answer any technical or syntax questions as to why this worked. Those questions would be better asked of others like the ones from the other discussion thread.
Happy to discuss Podio implementations (or anything else)!