September 15 – S Corporations
File a 2021 calendar year income tax return (Form 1120-S) and pay any tax due. This due date applies only if you requested an automatic 6-month extension. Provide each shareholder with a copy of their Schedule K-1 (Form 1120-S) or a substitute Schedule K-1 and, if applicable, Schedule K-3 (Form 1120-S) or substitute Schedule K-3 (Form 1120-S).
September 15 – Corporations
Deposit the third installment of estimated income tax for 2022 for calendar year
September 15 –  Social Security, Medicare and withheld income tax
If the monthly deposit rule applies, deposit the tax for payments in August.
September 15 – Nonpayroll Withholding
If the monthly deposit rule applies, deposit the tax for payments in August.
September 15 – Partnerships
File a 2021 calendar year return (Form 1065). This due date applies only if you were given an additional 5-month extension. Provide each partner with a copy of K-1 (Form 1065) or a substitute Schedule K-1.
September 30 – Fiduciaries of Estates and Trusts File a 2021 calendar year return (Form 1041). This due date applies only if you were given an extension of 5 1/2 months. If applicable, provide each beneficiary with a copy of K-1 (Form 1041) or a substitute Schedule K-1.