What is Ad Diem?

Legal Definition
At the day, as a plea of payment ad diem, on the day when the money became due. See Solvit ad diem, and Com. Dig. Pleader, 2 W. 29.
-- Bouviers Law Dictionary
Legal Definition
At a day; at the day. Townsh. PL 23. Ad certum diem, at a certain day. 2 Strange, 747. Solvit ad diem; he paid at or on the day. 1 Chit. PI. 485.
-- Black's Law Dictionary
Legal Definition
At a day.
-- Ballentine's Law Dictionary