backdate vs postdate

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. 

postdate

noun
  • A date on a document later than the real date on which it was written. 

verb
  • To affix a date to after the event. 

  • To occur after an event or time; to exist later on in time 

  • To assign an effective date to a document or action later than the actual date 

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