backdate vs foredate

backdate

verb
  • To give or assign a date to a document that is earlier than the current or true date. 

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

foredate

verb
  • To date before the true time; to antedate. 

How often have the words backdate and foredate occurred in a corpus of books? (source: Google Ngram Viewer )