Is it possible to create a field by Adding Date field + No of Days field

I am working with a customer in which specifically he wanted to add two fields like End Date (Forecasted) + Days Pending = test implementation end date, Here Days Pending is the number field he wants to add in Forecasted Date.

What we could do is, We could create a calculated field and it seems working but in this case we have to add Number value manually, however customer wants to get it added via field.