04 Sep September 15
Individuals: If you are not paying your 2017 income tax through withholding (or will not pay in enough tax during the year that way), pay the third installment of your 2017 estimated tax. Use IRS Form 1040-ES.
Employers: For Social Security, Medicare, withheld income tax, and non-payroll withholding, deposit the tax for payments in August if the monthly rule applies.
Corporations: Deposit the third installment of estimated income tax for 2017. Use the worksheet IRS Form 1120-W to help estimate tax for the year.
Partnerships: File a 2016 calendar-year return (IRS Form 1065). This due date applies only if you timely requested an automatic six-month extension. Provide each partner with a copy of their final or amended Schedule K1 (IRS Form 1065) or substitute Schedule K1 (IRS Form 1065).
S corporations: File a 2016 calendar-year income tax return (IRS Form 1120S) and pay any tax due. This due date applies only if you timely requested an automatic six-month extension. Provide each shareholder with a copy of Schedule K-1 (IRS Form 1120S) or a substitute Schedule K-1.