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Key Business Tax Deadlines for September 2025

As the tax landscape for September 2025 looms, businesses need to stay ahead by tracking crucial deadlines. This month’s significant dates include deadlines for S corporation and partnership tax filings with extensions, corporate estimated payments, and deadlines for payroll tax deposits. Let's delve into what needs to be done by September 15.

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September 15 - S Corporations
Businesses structured as S corporations must file their 2024 calendar year income tax return using Form 1120-S if they applied for a 6-month extension. It's crucial to distribute a Schedule K-1 (Form 1120-S) or its substitute to each shareholder, and if relevant, the Schedule K-3 or substitute Schedule K-3.

September 15 - Corporations
For calendar year corporations, September 15 marks the due date for depositing the third installment of estimated income tax for 2025.

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September 15 - Partnerships
Partnerships are required to file their 2024 calendar year return using Form 1065, with the same extension conditions as S corporations. Partners should receive their Schedule K-1 (Form 1065) or its alternative, as well as, when applicable, the Schedule K-3 (Form 1065) and its substitute.

September 15 - Social Security, Medicare, and Withheld Income Tax
Employers adhering to monthly deposit rules must deposit Social Security, Medicare, and withheld income tax for August 2025 by September 15. The same deadline applies to nonpayroll withholding deposits if following the monthly rule.

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Weekends & Holidays:
If any tax due date falls on a Saturday, Sunday, or legal holiday, it is automatically extended to the next business day that isn't a holiday.

Disaster Area Extensions:
For areas declared as disaster zones, deadlines are extended. To stay informed about such designations and their extended due dates, consult these resources:
https://www.fema.gov/disaster/declarations
https://www.irs.gov/newsroom/tax-relief-in-disaster-situations

At Thompson-Smith CPA, LLC, led by Georgia Smith, we provide tailored guidance to help businesses meet their tax obligations efficiently and accurately. Contact us for personalized assistance.

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