Here are some key tax-related deadlines for businesses and other employers during Quarter 3 of 2018. JULY 31: Report income tax withholding and FICA taxes for Q2 2018 (unless eligible for Aug. 10 deadline). File a 2017 calendar-year retirement plan report or request an extension. SEPT. 17: If a calendar-year partnership or S corp. that filed an extension, file a 2017 income tax return. If a calendar-year C corp., pay third installment of 2018 estimated income taxes.