September 2025 Essential Tax Deadlines for Businesses

As we navigate through September 2025, it's critical for businesses to stay on top of important tax deadlines. This month includes crucial filings for S corporations and partnerships, along with mandatory estimated corporate payments and payroll tax deposits, highlighting September 15 as a significant date for many companies.

September 15 - S Corporations
File your 2024 calendar year income tax return (Form 1120-S) and settle any taxes due on this date. This deadline applies if you’ve secured an automatic 6-month extension. Remember to furnish each shareholder with their Schedule K-1 (Form 1120-S) or an equivalent substitute, alongside Schedule K-3 (Form 1120-S) if applicable.

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September 15 - Corporations
Ensure the third installment of your estimated income tax for 2025 is deposited on this date, if you are operating under a calendar year framework.

September 15 - Partnerships
Submit your 2024 calendar year return (Form 1065) if a 6-month extension was previously granted. Partners should receive their Schedule K-1 (Form 1065) or a suitable alternative, including Schedule K-3 (Form 1065) where necessary.

September 15 - Social Security, Medicare, and Withheld Income Tax
If your business follows monthly deposit rules, ensure your deposit for August 2025's Social Security, Medicare, and withheld income tax is made by September 15. This deadline also applies for nonpayroll withholding deposits for August 2025.

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Handling Weekends & Holidays:
Keep in mind, if a deadline falls on a weekend or a holiday, it automatically shifts to the following business day.

Extensions for Disaster Areas:
In the event your region is declared a disaster area, deadline extensions are provided. For current details and specific extension dates, consult these resources:
FEMA: https://www.fema.gov/disaster/declarations
IRS: https://www.irs.gov/newsroom/tax-relief-in-disaster-situations

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