Essential Business Tax Deadlines for September 2025

The month of September 2025 is pivotal for businesses to meet key tax obligations. Topping the list are deadlines for S corporations and partnership filings, corporate estimated tax payments, and essential payroll tax deposits, all due by September 15. It's crucial that businesses remain vigilant to avoid penalties.

September 15 - S Corporations
Businesses structured as S corporations must file their 2024 calendar year income tax returns (Form 1120-S) by this date, provided they secured an automatic 6-month extension. Ensure each shareholder receives their Schedule K-1 (Form 1120-S) or a substitute, and if necessary, a Schedule K-3 (Form 1120-S) or substitute.

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September 15 - Corporations 

Corporations operating on a calendar year basis must deposit the third installment of their estimated income tax for 2025 by this deadline.

September 15 - Partnerships

For partnerships, the deadline to submit a 2024 calendar year return (Form 1065) is September 15, assuming a 6-month extension was granted. Partners should be furnished with their Schedule K-1 (Form 1065) or a substitute, and where applicable, a Schedule K-3 (Form 1065) or substitute.

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September 15 - Social Security, Medicare, and Withheld Income Tax
If you're an employer subject to monthly deposit rules, this is the due date for depositing Social Security, Medicare, and withheld income taxes for August 2025. The same date applies for nonpayroll withholding deposits if under monthly deposit rules.  

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Weekends & Holidays:

Should a due date fall on a Saturday, Sunday, or legal holiday, the deadline extends to the next business day not classified as a legal holiday. 

Disaster Area Extensions:
If your business is located in an area affected by disasters, filing deadlines may be extended. Confirm whether an area is designated a disaster zone by visiting the following official sources:
FEMA: https://www.fema.gov/disaster/declarations
IRS: https://www.irs.gov/newsroom/tax-relief-in-disaster-situations

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