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Key September 2026 Business Tax Deadlines

September brings several important tax deadlines for businesses, including extended returns for S corporations and partnerships, corporate estimated tax payments, and employer payroll tax deposits. At our firm in Billings, we believe in the “three-legged stool” of business stability: keeping your books accurate, your taxes optimized, and your payroll on time. Reviewing these key due dates will help keep your filings and payments on track so your service-based business has the solid support it needs to grow and weather challenges.

September 15: S Corporation Extended Returns

If your S corporation requested an automatic six-month extension, you must file your 2025 calendar year income tax return (Form 1120-S) and pay any tax due by September 15. You also need to provide each shareholder with a copy of their Schedule K-1 (Form 1120-S) or a substitute Schedule K-1, along with Schedule K-3 (Form 1120-S) or a substitute, if applicable.

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September 15: Corporate Estimated Taxes

Calendar year corporations are required to deposit their third installment of estimated income tax for the 2026 tax year by this date.

September 15: Partnership Extended Returns

For partnerships that timely requested an automatic six-month extension, September 15 is the deadline to file a 2025 calendar year return (Form 1065). Additionally, you must provide each partner with a copy of their Schedule K-1 (Form 1065) or a substitute, and Schedule K-3 (Form 1065) or a substitute, if applicable.

September 15: Payroll Tax Deposits

If you are an employer and the monthly deposit rules apply to your business, this is the due date to deposit Social Security, Medicare, and withheld income tax for August 2026. For those falling under the monthly deposit rule, this is also the deadline for the August 2026 nonpayroll withholding deposit. Keeping up with these deposits is a vital part of maintaining that on-time payroll leg of your business operations.

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September 30: Fiduciaries of Estates and Trusts

If you received a 5½-month extension, you must file a 2025 calendar year return (Form 1041) by the end of the month. If applicable, you must also provide each beneficiary with a copy of their Schedule K-1 (Form 1041) or a substitute Schedule K-1.

Weekend and Holiday Adjustments

If any tax due date falls on a Saturday, Sunday, or legal holiday, the deadline is automatically extended to the next business day that is not a legal holiday.

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Extensions for Disaster Areas

When a geographical area is designated as a disaster area, tax due dates are typically extended. For more information on whether your area has been designated as a disaster area and to review specific filing extension dates, visit the following resources:

Keep Your Business on Solid Footing

Managing these September due dates is crucial for maintaining compliance and optimizing your tax strategy. Whether you operate a subcontracting business, work in real estate, or run a service-based company across Montana or our neighboring states, staying ahead of these deadlines keeps your financial foundation strong. Contact our Billings office today to schedule a consultation and ensure your books, taxes, and payroll are fully supported.

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