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Essential Business Tax Deadlines for September 2025

As we move through September 2025, it's crucial for businesses to stay on top of upcoming tax deadlines. Key dates this month include deadlines for S corporations and partnerships filing on extensions, corporate estimated payments, and payroll tax deposits due on September 15. Ensuring compliance with these deadlines helps avoid penalties and keeps your financials on track.

September 15 - S Corporations
Calendar year S corporations should file their 2024 income tax return (Form 1120-S) and settle any taxes still owed by this date, given an automatic 6-month extension was requested. Ensure that each shareholder receives their Schedule K-1 (Form 1120-S), along with Schedule K-3 if applicable.

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September 15 - Corporations
Corporations operating on a calendar year must deposit the third installment of estimated income tax for the year 2025.

September 15 - Partnerships
For partnerships, the deadline to file a 2024 calendar year return (Form 1065) also falls on this date if a 6-month extension was previously obtained. Partners must receive their respective Schedule K-1 forms, with Schedule K-3s provided if needed.

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September 15 - Social Security, Medicare and Withheld Income Tax
Employers following the monthly deposit rules should complete their deposits of Social Security, Medicare, and withheld income tax for August 2025 by this due date. Nonpayroll withholding deposits for August 2025 also share this deadline under the monthly rule.

Weekends & Holidays:
If any deadline falls on a weekend or legal holiday, IRS rules automatically extend it to the next business day.

Disaster Area Extensions:
Areas designated as disaster zones receive filing extensions. For confirmation and additional information on whether your area qualifies and extended dates, visit these resources:
FEMA: https://www.fema.gov/disaster/declarations
IRS: https://www.irs.gov/newsroom/tax-relief-in-disaster-situations

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