A clean, keyword-rich deadline calendar for U.S. small business owners, S-corporation shareholders, and C-corporation owners: due dates, extensions (Form 7004), estimated taxes, and filing tips that help you avoid penalties and file your personal return on time.
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Key 2025 Corporate Deadlines (Calendar-Year Businesses)
| What | Due Date in 2025 | Notes |
|---|---|---|
| Form 1120-S (S-Corporation return) | Mar 17, 2025 | Normally Mar 15; moves to Monday since Mar 15 is a Saturday. Furnish Schedule K-1 to shareholders by this date. |
| Form 1120 (C-Corporation return) | Apr 15, 2025 | 15th day of the 4th month after year-end for calendar-year C-corps. |
| Form 7004 (automatic extension) — S-Corp | File by Mar 17, 2025 | Extends filing to Sept 15, 2025 (6 months). Extension is for paperwork, not payments. |
| Form 7004 (automatic extension) — C-Corp | File by Apr 15, 2025 | Extends filing to Oct 15, 2025 (6 months). Any tax due must be paid by Apr 15. |
| C-Corp Estimated Taxes (calendar year) | Apr 15 • Jun 16 • Sep 15 • Dec 15 | Quarterly installments for corporate income tax. S-corps generally don’t owe income tax, but may owe built-in gains or state/entity taxes. |
| Shareholder personal returns (Form 1040) | Apr 15, 2025 | K-1s from S-corps/partnerships feed into your individual return. If a K-1 is delayed, consider filing a personal extension. |
If a due date falls on a weekend or federal holiday, it shifts to the next business day.
Fiscal-Year Corporations
- S-Corps (1120-S): Due the 15th day of the 3rd month after the fiscal year ends. Example: year ending June 30 → due Sep 15.
- C-Corps (1120): Due the 15th day of the 4th month after year-end. Example: year ending Jan 31 → due May 15.
- June-30 C-Corp exception: Historically due in the 3rd month; some filers ending June 30, 2025 will still see a Sep 15, 2025 due date. Check your software/CPA if you use a June fiscal year.
Always confirm your fiscal-year settings in e-file software to generate the correct due date and voucher labels.
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Form 1120 vs. 1120-S: What Individual Owners Should Know
| Feature | Form 1120 (C-Corp) | Form 1120-S (S-Corp) |
|---|---|---|
| Who pays federal income tax? | Corporation pays entity-level tax | Pass-through to shareholders via Schedule K-1 |
| Owner’s personal filing impact | Dividends reported on Form 1040 | K-1 income/loss, credits flow to Form 1040 |
| Return due date (calendar year) | Apr 15, 2025 | Mar 17, 2025 |
| Extension (Form 7004) | To Oct 15, 2025 | To Sep 15, 2025 |
| Quarterly estimated taxes | Yes (Apr 15, Jun 16, Sep 15, Dec 15) | Generally no federal income tax; exceptions may apply |
If you own an S-corp, your personal return depends on timely K-1 delivery. Late corporate filing can force you to extend your Form 1040.
Owner’s 30-Day Checklist (Before Your Due Date)
- Close books and reconcile bank/credit accounts.
- Confirm depreciation schedules and Section 179/bonus elections.
- Verify shareholder basis (S-corp) and accumulated adjustments account.
- Review officer compensation, payroll filings, and state franchise/entity taxes.
- Schedule e-file authorization forms (e.g., 8879-S / 8879-C).
- Plan for cash to cover tax due (C-corp) or state/entity taxes (S-corp, where applicable).
- If not ready, file Form 7004 and pay your best estimate.
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