This also works, and as pointed out, is a little simpler:

DATEDIFF(dd, rec_dte, ISNULL(det_dte, GETDATE()))

Thanks again! 🙂

Bingo!

I used:

DateDiff(dd, rec_Dte, case when det_dte is null then GetDate() else det_dte end)

Results are returned as expected with no error.

Thank you very much!