Thanks Paul. I'd validated all the fields before, and re-done the code but couldn't get it working. Starting from scratch in a new document worked. Maybe it had genetic memory of an earlier error.

For reference, if I wanted to "set" this calculation to a variable, wa my original syntax workable, omitting the "quote" instead? E.g.

this worked for "storing" the difference between two dates but not for the 28 day calculation. I think it might be due to my lack of understanding of the use of "quote" in the original code.