What does "backdate" mean?

An assigned date that is earlier than the current or true date.

Backdate

Common synonyms include: antedate, antedate, antedated, antedates, backdated practice, backdating.

/ˈbæk.deɪt/ noun, verb

noun, verb ·Rare ·Advanced level

Noun

1
An assigned date that is earlier than the current or true date.
"The backdates on the clearance forms made the little man's papers worthless […]"

Verb

1
To give or assign a date to a document that is earlier than the current or true date.
2
make effective from an earlier date wordnet

"The backdates on the clearance forms made the little man's papers worthless […]"

From back + date.