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Essential Tax Deadlines: September 2025 for Businesses

As we navigate the complexities of business tax requirements, understanding the critical deadlines for September 2025 is paramount. Whether it’s filing S corporation or partnership extensions, making corporate estimated tax deposits, or managing payroll obligations, knowing the timelines helps ensure compliance and forestalls potential penalties.

September 15 - Key Date for S Corporations

Businesses structured as S corporations must file their 2024 calendar year income tax return (Form 1120-S) by this date, assuming an automatic six-month extension was previously requested. It's crucial to provide each shareholder with their Schedule K-1 (Form 1120-S) or an adequate substitute, alongside Schedule K-3 if necessary. This proactive step is vital for maintaining transparency and adherence to IRS mandates.

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Corporate Estimated Tax Payments

For corporations utilizing a calendar year, the third installment of estimated income tax for 2025 must be deposited by September 15. Ensuring timely payment keeps the business in good standing with the IRS and demonstrates diligence in financial planning.

Partnership Filing Deadlines

Partnerships, much like S corporations, are required to submit their 2024 calendar year return (Form 1065) by this deadline if an extension was procured. Partners should receive their Schedule K-1 (Form 1065) or a substitute version, and if applicable, Schedule K-3, securing compliance and clarity for all parties involved.

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Employer Responsibilities: Social Security, Medicare, and Withheld Income Tax

For employers obligated to follow monthly deposit rules, September 15 is the cutoff date to deposit taxes relating to August 2025 for Social Security, Medicare, and other withheld income taxes. Additionally, nonpayroll withholding deposits are due if the same monthly rule applies.

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Adjustments for Weekends & Holidays
If any deadline falls on a weekend or legal holiday, the due date is carried over to the next business day that is not a holiday. This frequent occurrence demands attention to avoid incorrect timing.

Filing Extensions in Disaster Areas
In instances where an area is declared a disaster zone, deadline extensions are granted. To verify if an area is affected and to find exact extension dates, resources such as FEMA and IRS websites offer up-to-date information.

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