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Critical September 2025 Tax Deadlines for Businesses

As we approach September 2025, it's crucial for businesses to mark their calendars for essential tax obligations. Key filing dates include S corporation and partnership filings with extensions, corporate estimated payments, and payroll tax submissions due by September 15.

September 15 - S Corporations
Ensure you file your 2024 calendar year income tax return (Form 1120-S) and settle any outstanding taxes by this date, presuming an automatic 6-month extension was requested. Additionally, provide each shareholder with their respective Schedule K-1 (Form 1120-S) or a substitute Schedule K-1, and where necessary, Schedule K-3 (Form 1120-S) or its substitute.

September 15 - Corporations
Corporations should deposit the third installment of their estimated income tax for 2025. This applies specifically to those operating on a calendar year basis.

September 15 - Partnerships
File the 2024 calendar year return (Form 1065), provided a 6-month extension was previously granted. Partners must receive a copy of their Schedule K-1 (Form 1065) or its substitute, as well as, if relevant, Schedule K-3 (Form 1065) or its substitute.

September 15 - Social Security, Medicare, and Withheld Income Tax
If monthly deposit rules are applicable, employers must deposit Social Security, Medicare, and withheld income tax for August 2025 by this deadline. Furthermore, the due date for the nonpayroll withholding deposit for August 2025 falls on this day if the monthly deposit rule applies.

Accommodations for Weekends & Holidays
Any deadline coinciding with a Saturday, Sunday, or legal holiday automatically extends to the next business day, provided it is not a holiday itself.

Extensions for Disaster Areas
Businesses located in designated disaster areas may qualify for deadline extensions. To verify if your area qualifies and for details on extended filing dates, visit these resources:
FEMA Disaster Declarations
Tax Relief in Disaster Situations - IRS

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