back·date
 /ˈbækˌdeıt/  verb  , -dates, -dat·ed, -dat·ing [  + obj  ]
  1 : to give (something) an earlier date than the actual date : antedate
   backdate a check
  2 : to say that something began or became effective at a date earlier than the current date
   an increase in salary backdated to the beginning of the year