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Key September 2025 Tax Deadlines for Businesses

As we step into September 2025, it's crucial for businesses to stay on top of essential tax obligations to ensure compliance and avoid penalties. This month includes significant due dates for S corporation and partnership tax filings, corporate estimated tax payments, and payroll tax deposits, all falling on September 15.

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September 15 - S Corporations
Prepare and file the 2024 calendar year income tax return (Form 1120-S) for your S corporation if you’ve availed an automatic 6-month extension. It's essential to furnish each shareholder with their respective Schedule K-1 (Form 1120-S) or its substitute. Additionally, don’t forget about Schedule K-3 (Form 1120-S) if it applies.

September 15 - Corporations
Corporate entities should deposit the third installment of their 2025 estimated income tax. Ensuring timely deposits keeps corporations compliant with the IRS guidelines.

September 15 - Partnerships
For partnerships, the 2024 calendar year return (Form 1065) needs filing provided there was a 6-month extension. Similar to S Corporations, ensure that each partner receives their Schedule K-1 (Form 1065) and if needed, the Schedule K-3 (Form 1065).

Image 1September 15 - Social Security, Medicare, and Withheld Income Tax
Employers adhering to the monthly deposit rules need to mark September 15 for their deposit of Social Security, Medicare, and withheld income tax for August 2025. It's also the cut-off for nonpayroll withholding deposits under the monthly deposit rule.

Be mindful of the Weekends & Holidays rule: if any due date falls on a non-business day, it extends to the next business day. Moreover, if you're in a Disaster Area, IRS deadlines will extend. Keep updated via FEMA or the IRS IRS website.

Understanding these due dates and the surrounding regulations can significantly aid in maintaining the financial health of your business.

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