· September 15 — S Corporations File a 2017 calendar year income tax return (Form 1120S) and pay any tax due. This due date applies only if you timely requested an automatic 6-month extension
· September 15 — Partnerships File a 2017 calendar year return (Form 1065). This due date applies only if you were given an additional 5-month extension
· September 15 — Corporations Deposit the third installment of estimated income tax for 2017
· September 15 — Social security, Medicare, and withheld income tax If the monthly deposit rule applies, deposit the tax for payments in August.