Datediff - column but not measure

[CahabaData]

A table had 2 date fields.  I could Datediff them to create a calculated column.  But when trying a measure of a Table visual - the Datediff would not work.  I could not select the table/field sources - they weren't presented by the intellisense and if I free form typed them in anyway it did not work.

I am trying to get a conceptual handle on the reason for this - in the broad sense involving potentially other DAX functions.  To understand in advance, rather than trial & error if it can be used as a Measure or not.