Key September 2025 Business Tax Deadlines

September 2025 is packed with critical tax deadlines for businesses, including filings for S corporations and partnerships under extensions, corporate estimated tax payments, and essential payroll tax deposits due by September 15.

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September 15 - S Corporations
File your 2024 calendar year income tax return (Form 1120-S) and ensure payment of any due taxes. Ensure compliance by distributing copies of Schedule K-1 (Form 1120-S) or a substitute, along with Schedule K-3 if necessary, to each shareholder.

September 15 - Corporations
Corporations following a calendar year must deposit their third installment of estimated income tax for 2025 on this date.

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September 15 - Partnerships
File the 2024 calendar year return (Form 1065) if you availed a 6-month extension. Issuance of Schedule K-1 (Form 1065) or a substitute, plus K-3 if applicable, to all partners is mandatory.

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September 15 - Social Security, Medicare, and Withheld Income Tax
For employers adhering to monthly deposit rules, this is the deadline to deposit Social Security, Medicare, and withheld income taxes for August 2025. Nonpayroll withholding deposits for August should also be made by this date.

Weekends & Holidays Compliance:
Should a deadline fall on a weekend or legal holiday, it extends automatically to the next business day that is not a holiday, minimizing compliance headaches.

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Disaster Area Filing Extensions:
In the event an area is declared a disaster zone, filing deadlines are extended. Stay informed on affected regions and new filing dates by visiting FEMA and IRS websites for up-to-date relief options.

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