BACKDATE
backdate
(verb) make effective from an earlier date; “The increase in tax was backdated to January”
Source: WordNet® 3.1
Etymology
Verb
backdate (third-person singular simple present backdates, present participle backdating, simple past and past participle backdated)
To give or assign a date to a document that is earlier than the current or true date.
Synonyms: antedate, Thesaurus:backdate
Noun
backdate (plural backdates)
An assigned date that is earlier than the current or true date.
Source: Wiktionary