Default Value Date Expressions

I have a field called 'date received' a field called 'internal deadline' and one called 'extrnal deadline'

The internal deadline is always 21 days after 'date received'.
The external deadline is always 41 days after 'date received'

In my table i want both the internal and external deadline fields to be auto generated when i enter a date in the 'date received' field.

eg. 'internal deadline'= 'date received' + 21 days.

I have tried the add date function, but that will not work.

I eventually want to create a working form, so that other users can enter this data with ease.
In this sense would it be easier to create this expression in form design rather then tabkle, if so how?