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Essential September 2025 Federal Tax Deadlines for Your Business

September is a crucial month for businesses as it brings key tax deadlines. Understanding these dates is essential for maintaining compliance and financial health. This month, keep an eye on the following due dates for S corporation and partnership filings with extensions, estimated corporate tax payments, and payroll tax deposits.

September 15 - S Corporations

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S Corporations must file their 2024 calendar year income tax return (Form 1120-S) and pay any tax due if they obtained an automatic six-month extension. Additionally, distribute Schedule K-1 (Form 1120-S) or a substitute Schedule K-1 to each shareholder, and if necessary, Schedule K-3 or its substitute.

September 15 - Corporations

The third installment of estimated income tax for 2025 is due for calendar year corporations. Make sure that payments are timely to avoid penalties.

September 15 - Partnerships

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Partnerships that received a six-month extension must file their 2024 calendar year return (Form 1065) and distribute a Schedule K-1 or a substitute to each partner. Likewise, include Schedule K-3 if applicable.

September 15 - Social Security, Medicare, and Withheld Income Tax

If subjected to monthly deposit rules, employers must deposit Social Security, Medicare, and withheld income tax for August 2025. This also includes nonpayroll withholding deposits for August 2025 under the same rule.

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Weekends & Holidays

In cases where a due date falls on a weekend or legal holiday, it is extended to the next business day that is not a legal holiday.

Disaster Area Extensions

Designated disaster areas can have their tax due dates extended. To determine if your area qualifies and for further details on filing extensions, visit these resources:

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