Form 2553 asks for the year end that will be used by the S corporation, which is usually December 31.  It also asks for the effective date of the S status.  This effective date has to be the beginning of the year that exists at the time of the election.  Since the C corporation has a year beginning October 1st, this is the effective date of the S status.  Therefore the short year from October 1st to December 31st will be the first S corporation status year.