backdate (v.)

"to or toward the rear or the original starting place; in the past; behind in position," literally or figuratively, late 14c., shortened from abak, from Old English on bæc "backwards, behind, aback" (see back (n.), and compare aback). To give (something) back is to give it again, to give it in the opposite direction to that in which it was formerly given. Adverbial phrase back and forth is attested by 1814.

c. 1400, daten, "to mark (a document) with a date," also "to assign to or indicate a date" (of an event), from date (n.1). Meaning "to mark as old-fashioned" is from 1895. Intransitive sense of "to have a date" is by 1850.