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Key Tax Deadlines for September 2025: Essential Dates for Businesses

September 2025 brings critical tax deadlines for businesses across various industries. Whether you are managing an S corporation, a partnership, or a corporate entity, being well-prepared for these due dates is vital for compliance and financial management. Here’s a breakdown of the key deadlines for September 2025, ensuring you can navigate your business obligations smoothly. Important Tax Deadlines

September 15 - S Corporations
For S corporations operating on a calendar year, September 15 is the deadline to file the 2024 Form 1120-S income tax return if you've secured an automatic 6-month filing extension. On this date, you must also ensure each shareholder receives their Schedule K-1 (Form 1120-S) or substitute, including Schedule K-3 if applicable. Compliance with these requirements helps sustain a transparent tax reporting process.

September 15 - Corporations
Corporations operating on a calendar basis need to deposit their third quarter estimated tax payments by September 15. This installment is critical for maintaining accurate and timely tax compliance throughout the fiscal year. Corporate Tax Preparation

September 15 - Partnerships
Partnerships granted a 6-month filing extension also face a September 15 deadline to file their 2024 calendar year Form 1065 return. Partners should receive their Schedule K-1 or substitute forms along with Schedule K-3 if needed, ensuring all parties are fully informed about their tax particulars.

September 15 - Social Security, Medicare, and Withheld Income Tax
Employers subject to monthly deposit rules must make their August 2025 deposits of Social Security, Medicare, and withheld income tax by this date. Additionally, any nonpayroll withholding for August 2025 needs to be deposited following the monthly deposit schedule. Adhering to these timelines ensures employer compliance with payroll tax obligations. Payroll Tax Calculation

Weekends & Holidays
If any due date coincides with a Saturday, Sunday, or legal holiday, it automatically extends to the next business day, preventing any last-minute scrambling.

Disaster Area Extensions
Should your business reside in a federally recognized disaster area, the IRS provides extensions for filing deadlines. For up-to-date details regarding disaster area designations and respective extensions, visit FEMA’s declaration page and the IRS tax relief page.

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