September 17
* Corporations: File calendar year Form 1120S if you timely requested a 6-month extension.
* Partnerships: File calendar year Form 1065 if you timely requested a 6-month extension.
* Individuals: Pay the third installment of your 2018 estimated tax - Use Form 1040-ES.
* Corporations: Deposit the third installment of your 2018 estimated tax.
* Electing Large Partnerships: File Form 1065-B if you timely requested a 6-month extension.