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Essential September 2025 Tax Deadlines for Business Owners

As autumn approaches, it brings with it critical tax deadlines that every savvy business owner should mark on their calendar. September is especially busy for those managing S corporations, partnerships, and corporate entities. Here’s a detailed breakdown to help you stay ahead.

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September 15 - S Corporations
It’s time to file your 2024 calendar year income tax return (Form 1120-S). This deadline applies if you took advantage of the automatic 6-month extension earlier this year. Don’t forget to furnish each shareholder with their Schedule K-1 (Form 1120-S). If needed, provide a Schedule K-3 (Form 1120-S) or a suitable substitute.

September 15 - Corporations
September 15 is also crucial for corporations making estimated payments. Deposit the third installment of your estimated income tax for the 2025 calendar year to stay compliant and avoid penalties.

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September 15 - Partnerships
For partnerships, submit your 2024 calendar year return (Form 1065) if you filed for a 6-month extension. Provide each partner with their respective Schedule K-1 (Form 1065), and where relevant, a Schedule K-3.

September 15 - Social Security, Medicare, and Withheld Income Tax
Employers must deposit withheld taxes for August 2025, including Social Security and Medicare. If the monthly deposit rule applies, ensure these deposits are timely to prevent any issues.

Handling Weekends & Holidays:
If any due date lands on a weekend or legal holiday, don’t panic—it automatically extends to the next business day. This provides a cushion for you to complete filings without worry.

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Extensions in Disaster Scenarios:
Should your business be in an area affected by a disaster, your filing deadlines may be extended. Stay informed by checking FEMA’s disaster declarations and the IRS’s tax relief page for updated information on your area.

For businesses in the Scottsdale, Denver, and Albuquerque regions, GeneralCents Accounting is here to help you navigate these deadlines. Our tech-savvy team provides strategic insight to ensure your financial peace of mind, so you can focus on what you do best—growing your business. Let’s turn that tax season stress into confidence and clarity.

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