A calendar tax year runs January 1 through December 31; a fiscal year is any other 12-month accounting period that ends on the last day of a month other than December. That is the whole distinction on paper. In practice, the fiscal year vs calendar tax year choice controls when your return is due, when estimated payments are scheduled, and which months of income land in which reporting period, and your entity type largely decides which option is even available to you.
The Two Definitions
Federal tax law defines a fiscal year as 12 consecutive months ending on the last day of any month other than December.1Office of the Law Revision Counsel. 26 USC 441 – Period for Computation of Taxable Income A retailer whose sales spike over the holidays might close its books on January 31 so all of that revenue and the resulting returns fall inside the same period. A company tied to an academic or agricultural cycle might pick June 30 or September 30 for the same reason.
A related option is the 52-53 week tax year. The business picks a specific day of the week (say, the last Saturday in March) as its year-end, and the actual closing date shifts by a day or two each year, alternating between 52 and 53 weeks. Manufacturers and retailers organized around weekly cycles often use this variant.1Office of the Law Revision Counsel. 26 USC 441 – Period for Computation of Taxable Income
The calendar tax year is the simpler of the two: January 1 through December 31. It is what most individual filers use because W-2s, 1099s, and bank statements already track the same window. The IRS requires a calendar year for any taxpayer who keeps no books, has no established annual accounting period, or whose accounting period does not qualify as a fiscal year.1Office of the Law Revision Counsel. 26 USC 441 – Period for Computation of Taxable Income That default catches nearly every sole proprietor and individual filer.
Who Gets to Choose
Whether you can adopt a fiscal year at all depends on your entity type.
C-Corporations
A C-corporation has the most latitude. It can adopt any fiscal year it wants when it files its first return, without prior IRS approval. Once that first return is filed, the chosen year is locked in, and changing it later requires formal IRS permission.
S-Corporations and Personal Service Corporations
S-corporations must use a “permitted year,” which defaults to December 31. The only way around that default is to convince the IRS that a different year-end serves a genuine business purpose, and shifting income into a later year for shareholders does not count as one.2Office of the Law Revision Counsel. 26 USC 1378 – Taxable Year of S Corporation Personal service corporations in fields such as health, law, accounting, and consulting face the same restriction: calendar year unless a genuine business purpose is established.1Office of the Law Revision Counsel. 26 USC 441 – Period for Computation of Taxable Income
Partnerships
Partnerships follow a cascade tied to the partners’ own tax years. A partnership must first adopt the year used by partners who together hold more than 50% of profits and capital. If no single year clears that threshold, it uses the year shared by all partners who each hold at least a 5% interest. Failing both tests, it defaults to the calendar year.3Office of the Law Revision Counsel. 26 USC 706 – Taxable Years of Partner and Partnership The point of the cascade is to keep partners from shifting income between periods to delay tax.
The Section 444 Workaround
S-corporations, partnerships, and personal service corporations that cannot demonstrate a business purpose still have one route to a fiscal year. Section 444 lets the entity adopt a fiscal year different from its required year, provided the gap between the two year-ends (the “deferral period”) is three months or less.4Office of the Law Revision Counsel. 26 USC 444 – Election of Taxable Year Other Than Required Taxable Year An S-corporation otherwise stuck with December 31 could elect September 30 this way.
There is a trade-off. Partnerships and S-corporations that make the election must submit an annual “required payment” using Form 8752, calculated from prior-year income and the deferral period. It functions as a deposit that offsets the tax benefit owners would otherwise get from the delay. Personal service corporations instead face limits on deductions for payments to employee-owners during the deferral period. Missing the Form 8752 deadline triggers a penalty of 10% of the underpayment.5Internal Revenue Service. Instructions for Form 8752
To elect, the entity files Form 8716 by the earlier of the 15th day of the fifth month following the start of the elected fiscal year, or the due date of the income tax return for that year.6eCFR. 26 CFR 1.444-3T – Manner and Time of Making Section 444 Election The election stays in effect until the entity changes its year-end or terminates the election, and once terminated, it cannot be made again.4Office of the Law Revision Counsel. 26 USC 444 – Election of Taxable Year Other Than Required Taxable Year
How Filing Deadlines Differ
The choice of year determines when the return is due. Calendar-year individuals and C-corporations file by April 15 following the close of the year.7Internal Revenue Service. When to File Fiscal-year individuals and C-corporations file by the 15th day of the fourth month after their year-end.8Office of the Law Revision Counsel. 26 USC 6072 – Time for Filing Income Tax Returns A C-corporation with a June 30 year-end therefore files by October 15.
Partnerships and S-corporations get an earlier deadline: the 15th day of the third month after year-end.8Office of the Law Revision Counsel. 26 USC 6072 – Time for Filing Income Tax Returns For calendar-year pass-throughs, that means March 15. The earlier date exists because those returns generate the Schedule K-1s partners and shareholders need for their own individual returns.9Internal Revenue Service. Publication 509, Tax Calendars
If the 15th falls on a Saturday, Sunday, or legal holiday, the deadline shifts to the next business day.7Internal Revenue Service. When to File Individuals can request a six-month extension on Form 4868; businesses use Form 7004 for the same six months.10Internal Revenue Service. About Form 7004 Both extend the time to file, not the time to pay. Interest on any unpaid balance starts running the day after the original deadline.11Internal Revenue Service. Form 4868 – Application for Automatic Extension of Time To File U.S. Individual Income Tax Return
Estimated Payments Under Each Year
Calendar-year taxpayers know the familiar quarterly dates: April 15, June 15, September 15, and January 15. Fiscal-year filers follow a parallel schedule pegged to their own year-end:
- First payment: 15th day of the 4th month of the fiscal year
- Second payment: 15th day of the 6th month
- Third payment: 15th day of the 9th month
- Fourth payment: 15th day of the 1st month after the fiscal year ends
You can skip the fourth payment if you file your return and pay the full balance by the last day of the first month after year-end.12Internal Revenue Service. Publication 505, Tax Withholding and Estimated Tax For a business with a March 31 year-end, the four dates would be July 15, September 15, December 15, and April 15.
Switching Between the Two
Once you have filed a return using a particular tax year, you cannot change without IRS permission. The process generally uses Form 1128, Application to Adopt, Change, or Retain a Tax Year.13Internal Revenue Service. About Form 1128 Some changes qualify for automatic approval under published IRS procedures and carry no user fee. Non-automatic changes, which need a formal IRS ruling, carry a user fee of $5,750 as of late 2025.14Internal Revenue Service. Internal Revenue Bulletin 2026-1 Professional preparation adds to that.
Non-automatic changes require proof of a genuine business purpose. The most common way to meet the standard is the natural business year test: if 25% or more of gross receipts consistently fall in the final two months of the requested year-end, measured over the three most recent years, the IRS will treat that period as the natural business year.15Internal Revenue Service. Instructions for Form 1128 A ski resort earning most of its revenue between November and March could likely support a March 31 year-end. What the IRS will not accept is a change whose primary effect is to defer income into a later tax year for the owners.2Office of the Law Revision Counsel. 26 USC 1378 – Taxable Year of S Corporation
An approved change usually forces one more filing: a short-period return covering the gap between the end of the old year and the start of the new one.15Internal Revenue Service. Instructions for Form 1128 Switching from December 31 to September 30, for example, produces a nine-month short-period return for January through September. Income on that return is annualized (multiplied by 12 and divided by the number of months in the short period), tax is computed on the annualized figure, and the result is prorated back to the short period.16eCFR. 26 CFR 1.443-1 – Returns for Periods of Less Than 12 Months The mechanism can push the same income into a higher bracket than a straight nine-month calculation would suggest. Individual filers on a short-period return also cannot claim the standard deduction, and any income-based credits are computed on the annualized figure rather than the raw short-period income.
Changing your tax year without approval is a mistake that compounds. The IRS can disregard the new period, recompute taxes using the original year-end, and assess back taxes plus interest for every year affected, often surfacing during an audit long after the change was made.