Essential September 2025 Tax Deadlines for Businesses

As we approach September 2025, business owners and accountants alike must keep an eye on key tax deadlines to ensure compliance and avoid penalties. Understanding these critical dates is essential for smooth financial management and safeguarding your business's fiscal health.

September 15 - S Corporations
Businesses structured as S Corporations need to file their 2024 calendar year income tax return (Form 1120-S) by this date if they have an automatic 6-month extension. Additionally, it's crucial to provide each shareholder with their Schedule K-1 (Form 1120-S) or a substitute. If required, supply Schedule K-3 for international tax details.Image 1

September 15 - Corporations
On this day, calendar year corporations must deposit the third installment of their 2025 estimated income tax. Ensuring timely payments helps maintain good standing with the IRS and avoids potential interest charges.

September 15 - Partnerships
For partnerships operating on a calendar year with a 6-month extension, the deadline for filing Form 1065 is September 15. As part of the filing, partners should receive a copy of their Schedule K-1 or its substitute, inclusive of any necessary Schedule K-3 details.Image 2

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September 15 - Social Security, Medicare, and Withheld Income Tax
Employers required to follow monthly deposit rules must deposit the withheld Social Security, Medicare, and income taxes for August 2025 by this deadline. This date also applies to nonpayroll withholding deposits.Image 3

Weekends & Holidays
In instances where a tax due date falls on a weekend or legal holiday, remember that the deadline extends to the next business day that isn't a holiday.

Disaster Area Extensions
Should your business be in a FEMA-designated disaster area, you may qualify for deadline extensions. For more detailed information, check out FEMA's disaster declarations and the IRS disaster relief resources.

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