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Essential Tax Deadlines: September 2025 for Businesses

Business owners and financial controllers should prepare for critical tax deadlines in September 2025, impacting S corporations, partnerships, and more. This month is pivotal for those who requested filing extensions and need to meet mandatory tax deposit requirements.

September 15 - Key Tax Filings for S Corporations
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For S corporations operating on a calendar year, it's time to file the 2024 income tax return (Form 1120-S) if you've availed the 6-month extension. Ensure that each shareholder receives their Schedule K-1 or a substitute, and where necessary, Schedule K-3 forms.

September 15 - Corporate Estimated Tax Deposits
September also requires corporations to deposit the third installment of estimated income tax for 2025. This is crucial for maintaining fiscal compliance and preparing for year-end financial assessments.

September 15 - Partnership Tax Filing Obligations
Partnerships with a calendar-year filing schedule must submit their 2024 tax returns (Form 1065) within this timeframe if a filing extension was granted. Partners should also receive their respective Schedule K-1 or alternate documentation, including Schedule K-3 where applicable.Image 3

September 15 - Payroll Tax Compliance
Employers subject to monthly deposit rules must make their Social Security, Medicare, and withheld income tax deposits for August 2025 by this date. Nonpayroll withholding deposits also adhere to this deadline under the same rules.

Understanding Special Circumstances

If a deadline coincides with a weekend or legal holiday, expect an automatic extension to the next workday that is not a legal holiday. Moreover, designated disaster areas may qualify for extended deadlines; visit the FEMA or the IRS websites for more information regarding these exceptions.

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