There is a very limited set of expressions you can use to calculate a field in a table. Note that I said "can". I feel safe in saying that the vast majority of experienced developers do not use calculated fields. You would be far better off having a date field and a frequency for something so that if you alter either value, the query for the form will always present the correct value. Otherwise, change one and you have to recalc for all existing records, which might prove very difficult to do reliably, if at all.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.