Sorry Bob, no luck with that.
Changing to "date_box_str" had the DLookup looking for values "txtDay1", "txtDay2" etc from "tmp_tblHours" when the field values I need are dates.
I've also tried the below without any luck...
hour_box.Value = DLookup("[Hours Worked]", "tmp_tblHours", "[when] = #"...