backdate
verb [ T ]
/ˌbækˈdeɪt/ /ˈ--/ ukusto make something effective from a date earlier than the present date:
They agreed that the pay increase would be backdated to July.
Those signing up before October will have their pension payments backdated.
to put a date on a document that is earlier than the date when you wrote it:
The jury's verdict recognized that backdating options with intent to deceive is a crime.
backdate a cheque
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post-date