Short year tax return due dates fall on the 15th day of a specific month after the short period ends, and which month depends on the entity. C-corporations, estates, and trusts file on the 15th day of the fourth month. S-corporations and partnerships file on the 15th day of the third month. The same timing applies whether the short year came from a change in accounting period, a new entity’s first year, a dissolution, or a subsidiary joining a consolidated group.1Office of the Law Revision Counsel. 26 USC 443 – Returns for a Period of Less Than 12 Months
Due Dates by Entity Type
C-Corporations (Form 1120)
A C-corporation’s short-period return is due on the 15th day of the fourth month after the short period ends.2Internal Revenue Service. Instructions for Form 1120 A short year ending September 30 means a January 15 deadline. A C-corporation that dissolves on June 15 owes its final return by October 15.
S-Corporations (Form 1120-S) and Partnerships (Form 1065)
S-corporations and partnerships file one month earlier than C-corporations: the 15th day of the third month after the short period closes.3Internal Revenue Service. Instructions for Form 1120-S (2025)4Internal Revenue Service. Instructions for Form 1065 (2025) – Section: When To File A partnership with a short year ending September 30 must file by December 15. An S-corporation that dissolves on March 20 owes its final return by June 15.
Multi-member LLCs that haven’t elected corporate taxation follow the same schedule as partnerships.4Internal Revenue Service. Instructions for Form 1065 (2025) – Section: When To File
Estates and Trusts (Form 1041)
Estates and trusts follow the C-corporation timeline: the 15th day of the fourth month after the short period ends.5Internal Revenue Service. Forms 1041 and 1041-A: When to File An estate with a short year ending June 30 would file by October 15.
Deceased Individuals (Form 1040)
When someone dies mid-year, the final individual return covers January 1 through the date of death. The deadline stays on the normal April 15 schedule for the year of death, even though the period is short.6Internal Revenue Service. Filing a Final Federal Tax Return for Someone Who Has Died The surviving spouse or personal representative files it. Any estate the person leaves behind has its own separate filing obligation on Form 1041.
Weekends and Holidays
If a short-period due date falls on a Saturday, Sunday, or legal holiday, the deadline shifts to the next business day.7Internal Revenue Service. When to File
When a Short Tax Year Applies
A short tax year is any federal income tax reporting period covering fewer than 12 full months. Two situations create one: a change in annual accounting period, or an entity that didn’t exist for the full year.1Office of the Law Revision Counsel. 26 USC 443 – Returns for a Period of Less Than 12 Months
The most common trigger is switching accounting periods. A business moving from a calendar year ending December 31 to a fiscal year ending September 30 files a stub return covering the gap. New corporations and partnerships formed mid-year file a short first-year return from formation through their chosen year-end. Businesses that dissolve or liquidate before their normal year-end file a short final return for the portion of the year they operated.
A less obvious trigger: a subsidiary joining a consolidated group. Its tax year ends on the day it becomes a member, producing a short period that requires a separate return.8eCFR. 26 CFR 1.1502-76 – Taxable Year of Members of Group
A short-period return uses the same form you would file for a full year. Fill in the “Tax Year Beginning” and “Tax Year Ending” fields to reflect the actual short period, and mark it as a final return if the entity is closing. When the short year comes from a voluntary change in accounting period, most taxpayers also need to file Form 1128 to request IRS approval, by the due date of the short-period return including extensions.9Internal Revenue Service. About Form 1128, Application to Adopt, Change or Retain a Tax Year
Extensions
Form 7004 grants an automatic extension on short-period business returns the same way it does on full-year returns. File it on or before the original due date of the short-period return.10Internal Revenue Service. About Form 7004, Application for Automatic Extension of Time To File Certain Business Income Tax, Information, and Other Returns
The extension length depends on the entity:
- Corporations, partnerships, and S-corporations get six months from the original deadline.
- Estates and trusts filing Form 1041 get five and a half months.
Both figures come from the Form 7004 instructions.11Internal Revenue Service. Instructions for Form 7004 (12/2025) A C-corporation with a short-year return originally due November 15 would have until May 15 of the following year. An S-corporation originally due December 15 would push to June 15.
An extension buys time to file, not time to pay. Any estimated tax must still be paid by the original due date, or interest and the failure-to-pay penalty start accruing even if the return itself lands within the extension window. Form 7004 can be submitted electronically or on paper and requires the entity’s EIN and the correct short-period ending date.
Penalties for Missing the Deadline
Short-period penalties match those on a full-year return. They are worth knowing because the usual March or April muscle memory doesn’t apply, and non-standard due dates slip past easily.
Failure-to-File Penalty
For C-corporations filing Form 1120, the penalty is 5% of unpaid tax per month or partial month, capped at 25%. If the return is more than 60 days late, the minimum penalty is $525 or 100% of the unpaid tax, whichever is less.12Internal Revenue Service. Failure to File Penalty
Partnerships and S-corporations use a per-owner calculation: $255 per partner or shareholder for each month the return is late, up to 12 months. A 10-member partnership that files four months late owes $10,200 ($255 × 10 × 4).12Internal Revenue Service. Failure to File Penalty The math compounds quickly, which is why nailing the short-year deadline matters especially for pass-through entities.
Failure-to-Pay Penalty
If tax is owed but unpaid by the original due date, the IRS charges 0.5% of the unpaid balance per month, up to 25%. When failure-to-file and failure-to-pay apply in the same month, the failure-to-file rate drops to 4.5% so the combined penalty is 5% per month.13Internal Revenue Service. Failure to Pay Penalty
Interest
Interest accrues on unpaid tax from the original due date and compounds daily. For the quarter beginning April 1, 2026, the underpayment rate is 6% for most taxpayers and 8% for large corporate underpayments.14Internal Revenue Service. Internal Revenue Bulletin: 2026-08 Rates reset quarterly, so the figure in effect when your short-period return is due may be different.