- If a calendar-year C corporation that filed an automatic six-month extension:
- File a 2017 income tax return (Form 1120) and pay any tax, interest, and penalties due.
- Make contributions for 2017 to certain employer-sponsored retirement plans.
- Report income tax withholding and FICA taxes for third quarter 2018 (Form 941) and pay any tax due. (See exception below under “November 13.”)
- Report income tax withholding and FICA taxes for third quarter 2018 (Form 941), if you deposited on time and in full all of the associated taxes due.
- If a calendar-year C corporation, pay the fourth installment of 2018 estimated income taxes.