This same question was recently asked on TUG. The response was to use
www.timeanddate.com/date/dateadd.html. Use the form to simply subtract 331 days from your proposed flight date. Good luck. Note that the online booking doesn't show 331 days out most of the time, usually it's behind by about two days (United, anyway). I always call at the exact time the flights are released to try and get the high demand flights.