backdate

[ bak-deyt ]
/ ˈbækˌdeɪt /

verb (used with object), back·dat·ed, back·dat·ing.

to date earlier than the actual date; predate; antedate: Backdate the letter so he'll think I wrote it last week.

Origin of backdate

An Americanism dating back to 1945–50; back2 + date1

British Dictionary definitions for backdate

backdate
/ (ˌbækˈdeɪt) /

verb

(tr) to make effective from an earlier date the pay rise was backdated to August