Add Integer to Date Field

We are building an application to track the gathering of requirements in Fibery.
We have an entity called Project and it currently has a start and end date, they can all be displayed on a timeline. That’s good.
I’d love it to be a bit more dynamic though.
I can add a field for forecast project duration to hold an integer value in days or weeks
I’d love to calculate an end date from the start date + duration but Fibery won’t allow me to do that ( I think )
Is there a formula that I’ve missed?

Thanks in advance

I actually found a solution this morning, straightforward enough

[Forecast Start Date] + Days([Forecast Duration])

Happy days
On to the next problem