Updated October 17, 2026. Quick answer: Form 1065 is the partnership return and Form 1120-S is the S corporation return, and both are due on the 15th day of the 3rd month after the tax year ends. Applying that rule, a calendar year 2026 return is due Monday, March 15, 2027, a date computed here and not published by the IRS. Form 7004 gives an automatic 6 months to file, to September 15, 2027, but it does not extend the time to pay tax, and each partner or shareholder is owed a Schedule K-1.
S corporation return (Form 1120-S) for 2025, read line by line: Form 1120-S lines 1a to 6, Form 1120-S lines 23 to 28 and Form 1120-S Schedule M-2.
Partnership return (Form 1065) for 2025, read line by line: Form 1065 lines 24 to 31, Form 1065 Schedule B questions 4 and 5 and Form 1065 Schedule B questions 25 to 33.
The dates at a glance
The editions read are written for the 2025 return, so they print 2026 dates. March 15, 2026 was a Sunday, which is why the instructions print March 16. The table pairs each printed date with the same rule applied one year later.
| Date | As printed for the 2025 return | Same rule for the 2026 return |
|---|---|---|
| Form 1065, calendar-year partnership | “For calendar-year partnerships, the due date is March 15.” “Calendar-year partnerships may therefore timely file their returns for the 2025 partnership year by March 16, 2026.” | March 15, 2027, a Monday (computed here, not published by the IRS) |
| Form 1120-S, calendar-year S corporation | “For calendar year corporations, the due date is March 16, 2026 (March 15th falls on Sunday).” | March 15, 2027, a Monday (computed here, not published by the IRS) |
| Extended date for either form | “file the return and provide each partner with a copy of their final or amended (if required) Schedule K-1 (Form 1065)” and “by September 15.” | September 15, 2027, a Wednesday (computed here, not published by the IRS) |
When Form 1065 is due
The Form 1065 instructions state the rule: “Generally, a domestic partnership must file Form 1065 by the 15th day of the 3rd month following the date its tax year ended” and, as the table shows, for a calendar year that is March 15.
The weekend rule is printed in the same section: “If the due date falls on a Saturday, Sunday, or legal holiday in the District of Columbia or the state in which you file your return, a return filed by the next day that isn’t a Saturday, Sunday, or legal holiday will be treated as timely.”
March 15, 2027 falls on a Monday, so for a calendar year 2026 partnership the rule does not move the date. That date is computed here, not published by the IRS. See the Form 1065 page for the form itself.
When Form 1120-S is due
The S corporation rule is the same length: “Generally, an S corporation must file Form 1120-S by the 15 day of the 3rd month after the end of its tax year.” “If the due date falls on a Saturday, Sunday, or legal holiday, the corporation can file on the next day that isn’t a Saturday, Sunday, or legal holiday.”
Applied to a calendar year 2026 return, the due date is Monday, March 15, 2027, computed here, not published by the IRS. See the Form 1120-S page for the form itself.
The 6 month extension on Form 7004
Both sets of instructions point to the same extension form. The Form 1065 instructions say “File Form 7004 by the regular due date of the partnership return.” The Form 1120-S instructions say “Generally, the corporation must file Form 7004 by the regular due date of the return.”
The Form 7004 instructions give the length: “The automatic extension period for time to file is generally 6 months.” “Properly filing Form 7004 will automatically give you the maximum extension allowed from the due date of your return to file the return.”
There is no signature and no separate form for each partner. The instructions say “No signature is required on this form.” and “File a separate Form 7004 for each return for which you are requesting an extension of time to file.”
Publication 509 puts the extended date on its calendar for partnerships and S corporations: “This due date applies only if you timely requested an automatic 6-month extension.” For a calendar year 2026 return, 6 months after March 15, 2027 is September 15, 2027, a Wednesday, computed here, not published by the IRS.
What the extension does not extend
The Form 7004 instructions open the payment section with a caution: “Form 7004 does not extend the time to pay any tax due.” “Interest is charged on any tax not paid by the regular due date of the return from the due date until the tax is paid.”
Publication 509 adds a deposit step for an S corporation extension: “file Form 7004 and deposit what you estimate you owe in tax.”
A paper request has a filing risk too: “If you file Form 7004 on paper and file your tax return electronically, your return may be processed before the extension is granted. This may result in a penalty notice.”
Schedule K-1 to partners and shareholders
The K-1 is part of the filing. The Form 1065 instructions say “Attach a copy of each Schedule K-1 to the Form 1065 filed with the IRS.” “furnish a copy to each partner.” The Form 1120-S instructions say “Attach a copy of each Schedule K-1 to the Form 1120-S filed with the IRS.” “give each shareholder a copy.”
The calendar ties the copy to the same date as the return. Publication 509 says, for partnerships, “Provide each partner with a copy of their Schedule K-1 (Form 1065)” and for S corporations “Provide each shareholder with a copy of their Schedule K-1 (Form 1120-S)”. On extension, the same publication says the final or amended copy is due by September 15, which is September 15, 2027 on the computed calendar above. For what each box means, see the Schedule K-1 Form 1065 boxes page.
Penalties as printed
The late filing penalty for a partnership is stated per partner, and for an S corporation per shareholder. Reasonable cause is a defense in both sets of instructions.
| Penalty | As printed |
|---|---|
| Form 1065 late filing | “The penalty is $255 for each month or part of a month (for a maximum of 12 months) the failure continues, multiplied by the total number of persons who were partners in the partnership during any part of the partnership’s tax year” |
| Form 1120-S late filing, no tax due | “the penalty is $255 for each month or part of a month (up to 12 months) the return is late or doesn’t include the required information, multiplied by the total number of persons who were shareholders in the corporation” |
| Form 1120-S late filing, tax due | “If tax is due, the penalty is the amount stated above plus 5% of the unpaid tax for each month or part of a month the return is late, up to a maximum of 25% of the unpaid tax.” |
| Form 1120-S late payment | “A corporation that doesn’t pay the tax when due may generally be penalized 1/2 of 1% of the unpaid tax for each month or part of a month the tax isn’t paid, up to a maximum of 25% of the unpaid tax.” |
The K-1 has its own penalty. The Form 1065 instructions say “a $340 penalty may be imposed for each Schedule K-1” for which a failure to furnish occurs, and the Form 1120-S instructions say the same amount “may be imposed with respect to each Schedule K-1”. The Form 1120-S instructions add “The penalty won’t be imposed if the corporation can show that not furnishing information timely was due to reasonable cause.”
The editions read for this page are the 2025 Instructions for Form 1065, the 2025 Instructions for Form 1120-S, the Instructions for Form 7004 (Rev. December 2025) and IRS Publication 509, Tax Calendars (2026), which are the revisions irs.gov served when they were read on 2026-10-17. They are written for the 2025 return filed in 2026, so every 2027 date on this page is computed here by applying the printed rules and is not published by the IRS. Check the revision printed on the form you are filing, because a later revision can change the dates and the penalty amounts.
Common mistakes
- Treating Form 7004 as more time to pay. The Form 7004 instructions say it does not extend the time to pay any tax due, and interest is charged on tax not paid by the regular due date.
- Filing Form 7004 late. The instructions say it must be filed by the regular due date of the return, and the Form 1065 and Form 1120-S instructions repeat that.
- Holding the K-1s until the return is done on an extended filing. Publication 509 pairs each K-1 with the return date, and the instructions print a per-K-1 penalty for failing to furnish one when due.
Related: Form 1065, Form 1120-S, Schedule K-1 Form 1065 boxes, Form 1041 due date and extension, and Form 709 due date and extension.
Sources
- 2025 Instructions for Form 1065: https://www.irs.gov/pub/irs-pdf/i1065.pdf
- 2025 Instructions for Form 1120-S: https://www.irs.gov/pub/irs-pdf/i1120s.pdf
- Instructions for Form 7004 (Rev. December 2025): https://www.irs.gov/pub/irs-pdf/i7004.pdf
- IRS Publication 509, Tax Calendars (2026): https://www.irs.gov/pub/irs-pdf/p509.pdf
Read from the IRS documents linked above on October 17, 2026. General information, not tax advice. Your own facts decide the outcome, and a preparer, the account provider or the IRS is the right place to confirm anything consequential.