According to TurboTax, for a rejected return:
October 22, 2017 at 3 AM PST- Last day to fix rejected returns and not incur additional penalties or interest (again assuming an extension was filed and accepted)
Note that that is barely into October 22. TurboTax is sometimes saying "October 21" to be sure people don't think they can efile during the daytime of October 22.
