For a calendar-year client, the partnership and S corporation returns land in the third month after the year ends and the C corporation return in the fourth, which put the 2025 returns on March 16 and April 15, 2026. Those are the two dates a firm plans around. The rest of the IRS business tax filing deadlines sit on the same published calendar and rarely reach a scheduling board: the Form 7004 periods that are not six months, the quarterly employment tax returns, and the corporate estimated tax installments.
IRS Business Tax Filing Deadlines for Calendar-Year Clients
Start with the clients on a calendar year. These are the dates for a 2025 calendar-year return, and they are already adjusted, because the tax calendar makes the adjustments for Saturdays, Sundays and legal holidays itself, which is why the partnership row reads March 16 rather than the fifteenth (IRS Publication 509 (2026), Tax Calendars).
The legal holidays it adjusts for are the District of Columbia's, and you still have to make any adjustment for a statewide legal holiday yourself. That kind of holiday delays a filing due date only if the IRS office where you are required to file is located in that state, and it never delays a federal tax deposit (IRS Publication 509 (2026), Tax Calendars).
| Return | Filed by | Due in 2026 | Extended due date |
|---|---|---|---|
| Form 1065 | Partnership | March 16 | September 15 |
| Schedule K-1 (Form 1065) | Partnership, to each partner | March 16 | September 15 |
| Form 1120-S | S corporation | March 16 | September 15 |
| Schedule K-1 (Form 1120-S) | S corporation, to each shareholder | March 16 | September 15 |
| Form 1120 | C corporation | April 15 | October 15 |
| Schedule C (Form 1040) | Sole proprietor or single-member LLC | April 15 | October 15 |
| Source: IRS Publication 509 (2026), Tax Calendars. |
The K-1 rows are not downstream of the return rows. They carry the same date. A Schedule K-1 is the statement reporting each partner's or shareholder's share of the entity's income, deductions and credits, and its copy is due on the return's own date rather than after it, with the extended return carrying the final or amended copy with it. What that does to a book whose year ends all land in one window is worked through in year-end accounts outsourcing.
The Schedule C row is not an entity return, and a business-tax checklist tends to drop it. Schedule C (Form 1040) reports the income or loss from a business you operated or a profession you practiced as a sole proprietor (IRS, About Schedule C (Form 1040)), and a single-member LLC that has not elected to be treated as a corporation is a disregarded entity whose activities are generally reflected on its owner's Schedule C (IRS, Single Member Limited Liability Companies). Both run on the individual return's dates, which the tax calendar puts at April 15 with an extension to October 15 (IRS Publication 509 (2026), Tax Calendars).
The Same Rule Produces Different Dates for a Fiscal-Year Client
A fiscal-year client is not an exception to the calendar. It is the same counting rule started from a different month. Form 1065 and Form 1120-S are due on the 15th day of the 3rd month after the end of the tax year, and Form 1120 on the 15th day of the 4th month (IRS Publication 509 (2026), Tax Calendars). What a firm needs on a schedule is the arithmetic done, so here are four real year ends and the dates they produce.
| Client tax year ends | Return | Return due | Extended due date |
|---|---|---|---|
| June 30, 2026, partnership | Form 1065 | September 15, 2026 | March 15, 2027 |
| September 30, 2026, S corporation | Form 1120-S | December 15, 2026 | June 15, 2027 |
| March 31, 2026, C corporation | Form 1120 | July 15, 2026 | January 15, 2027 |
| June 30, 2026, C corporation short year begun before January 1, 2026 | Form 1120 | September 15, 2026 | April 15, 2027 |
| Source: IRS Publication 509 (2026), Tax Calendars for the due dates and IRS, Instructions for Form 7004 for the extension periods. A date landing on a Saturday, Sunday or legal holiday generally moves to the next day that is none of those. |
The last row's return date is published rather than derived, and it is the one a scheduling board gets wrong. A short tax year is a tax year of less than 12 months, which a client gets when it is not in existence for an entire tax year or changes its accounting period (IRS Publication 538).
Corporations with a short tax year that began before January 1, 2026 and ends on June 30, 2026 must file by September 15, 2026, and a corporation with a short tax year ending anytime in June is treated as if the short year ended on June 30 (IRS Publication 509 (2026), Tax Calendars).
Form 7004 Covers Several Returns and Grants Three Different Periods
Form 7004 is one application used to request an automatic extension of time to file certain business income tax, information and other returns, and a separate Form 7004 goes in for each return rather than one covering a group of them (IRS, Instructions for Form 7004). The period it grants is not the same length for every filer.
| Return | Automatic extension period |
|---|---|
| Form 1065, Form 1120-S and Form 1120, general rule | 6 months |
| Form 1120, C corporation tax year ending June 30 and beginning before January 1, 2026 | 7 months |
| Form 1120-POL, the political organization return, that same June 30 tax year | 6 months |
| Form 1041, the estate and trust income tax return, for an estate other than a bankruptcy estate or for a trust | 5½ months |
| Source: IRS, Instructions for Form 7004. |
The June 30 row has an end date attached to it. C corporations with tax years ending June 30 and beginning before January 1, 2026 are eligible for the 7-month extension, and for tax years beginning in 2026 the automatic extension period is 6 months (IRS, Instructions for Form 7004).
What that cutoff moves is the return date, not the extended one. Form 7004 runs its period from the due date of the return (IRS, Instructions for Form 7004), and the two return dates are not the same. A short year that began before January 1, 2026 and ended June 30, 2026 is due September 15, 2026, and 7 months from there is April 15, 2027. A short year that began in 2026 and ended June 30, 2026 is due on the 15th day of the 4th month, October 15, 2026, and 6 months from there is the same April 15, 2027 (IRS Publication 509 (2026), Tax Calendars).
Mark which of your June clients sit on each side of that cutoff, because what it changes is when the return is due, not the month the extension ends.
One thing the form does not do is move the money. Form 7004 does not extend the time to pay any tax due, and how a workflow record has to represent that split without losing the payment date is worked through in accounting workflow software.
Employment Tax Returns Run on Their Own Quarters
The employment tax calendar does not line up with the income tax calendar, and it is easy to leave off a schedule built around return dates. Form 941, the quarterly federal employment tax return, is filed for each quarter by the last day of the month that follows the end of the quarter, which the IRS publishes as April 30, July 31, October 31 and January 31. Form 940, the annual federal unemployment tax return, reports the previous year and has a due date of January 31. If you timely deposited all taxes when due, you have 10 additional calendar days to file either return (IRS, Employment tax due dates).
Applied to a real year, with the weekend adjustments already made, that gives the following.
| Return | Period covered | Due in 2026 | If deposits were made timely, properly and in full |
|---|---|---|---|
| Form 941 | Fourth quarter of 2025 | February 2 | February 10 |
| Form 941 | First quarter of 2026 | April 30 | May 11 |
| Form 941 | Second quarter of 2026 | July 31 | August 10 |
| Form 941 | Third quarter of 2026 | November 2 | November 10 |
| Form 940 | Calendar year 2025 | February 2 | February 10 |
| Source: IRS Publication 509 (2026), Tax Calendars. |
Two neighboring workloads sit on their own dates and belong on the same board. Deposit due dates are a separate schedule, fixed before the year begins rather than by the pay run, and they are worked through in the deposit schedule a payroll engagement runs on. The turn-of-year information-return run, Forms 1099-NEC, W-2 and W-3 among them, which the same weekend shift puts on February 2 in 2026 (IRS Publication 509 (2026), Tax Calendars), is a distinct annual job set out in year-end accounting services.
Corporate Estimated Tax Adds Four More Dates
A calendar-year corporation deposits its estimated income tax for the year in four installments, and for 2026 those fall on April 15, June 15, September 15 and December 15. A fiscal-year corporation pays on the 15th day of the 4th, 6th, 9th and 12th months of its own tax year (IRS Publication 509 (2026), Tax Calendars). The first installment shares a day with the Form 1120 filing deadline, so one April date carries two obligations for the same client, one looking backward and one forward.
A schedule built only from return dates shows the filing obligation and hides the deposit. Put the installment dates on the same board as the filing dates, marked as the client's, not the preparer's.
Put the Calendar Somewhere the Work Can See It
Take your client list and mark four things against every name: the return it files and its date, whether a K-1 obligation comes out of it, whether the year end is anything other than December, and whether the client files Form 941. Check the last two columns first. They are the ones that produce a date nobody typed in.
Then look at what happens when the extended dates arrive with the spring's deferred work still attached to them, which is a sequencing problem before it is a staffing one. Once the sequence is set, what is left is capacity, and that belongs with capacity planning, while what an empty seat standing in front of one of these dates actually costs is priced in the cost of employee turnover.
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