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Essential September 2025 Tax Deadlines for Businesses

September 2025 is a critical month for businesses in terms of tax reporting and payments. Key deadlines approach for S corporations, partnerships, and corporations, making it essential for tax professionals and business owners alike to stay ahead of their obligations. Failure to comply can lead to significant penalties. Here’s a breakdown of September’s crucial tax due dates to ensure you’re prepared.

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September 15 - S Corporations
S corporations must file their 2024 calendar year tax return (Form 1120-S) by this date if they've requested the automatic 6-month extension. Ensure that each shareholder receives their Schedule K-1 (Form 1120-S) or a substitute, and where applicable, Schedule K-3. Preparing these schedules accurately is vital for reflecting each shareholder’s portion of the income, deductions, and credits.

September 15 - Corporations
Corporations need to deposit their third installment of estimated income tax for the year 2025. This installment is crucial for those operating on a calendar year, helping to manage cash flow and comply with IRS requirements.

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September 15 - Partnerships
Partnerships are required to file their 2024 calendar year return (Form 1065) by this date if they obtained a 6-month filing extension. Like S corporations, they need to provide partners with their respective Schedule K-1 (Form 1065), and where necessary, Schedule K-3 or a substitute.

September 15 - Social Security, Medicare, and Withheld Income Tax
For employers bound by the monthly deposit rules, this date also marks the deadline to make deposits for Social Security, Medicare, and withheld income tax for August 2025. Additionally, it covers the nonpayroll withholding deposit if the monthly rules apply.

Weekends & Holidays
If any due dates fall on a weekend or legal holiday, the IRS provides flexibility by extending the due date to the next business day. Understanding this rule can help avoid last-minute panic and ensure timely compliance.

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Disaster Area Extensions
In cases where a region is declared a disaster area, tax deadlines may be extended, offering relief to affected businesses. To check if your area qualifies, visit FEMA's Disaster Declarations or the IRS’s Tax Relief in Disaster Situations.

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