Here are a few key tax-related deadlines for businesses and other employers during Quarter 4 of 2018. OCT. 15: If a calendar-year C corp. that filed an extension, file a 2017 income tax return. OCT. 31: Report income tax withholding and FICA taxes for Q3 2018 (unless eligible for Nov. 13 deadline). DEC. 17: If a calendar-year C corp., pay fourth installment of 2018 estimated income taxes. Contact us for more about the filing requirements and to ensure you’re meeting all applicable deadlines.