2017 rate for your 2017 return. as you said, A fiscal tax year f your C corp is a period of 12 consecutive months ending on the last day of any month other than December, or a 52?53 week period that ends on a specific day of the week occurring either in the last week or nearest the last day of a specific month.Then, you neede to file your C corp t\return of 1120 is due on the 15th day of the third month following the end of the corporation's tax year. For example, corporations with a fiscal year ending March 31 must file their tax returns by June 15. Corps whose fiscal year ends June 30 must file by Sep. 15; fiscal years ending Sep. 30, by Dec. 15. Any tax due must be paid at this time.