Many thanks to both you and Ron for taking the time to reply and report this
error. I just sent an interim response before I realized you'd responded.
This reply stated that the error seems to center around that end date range
only (as far as I could tell).

Anyway, thanks again.

As a P.S. : the cpearson link did have the following quote which
underscores Ron's comments:"DATEDIF is treated as the drunk cousin of the
Formula family."