Startup tax deadlines by month: the calendar for a Delaware C-corp

01Every month#

Before the calendar, the recurring work that most deadlines depend on:

• Close the books for the prior month, ideally within the first five to ten business days, so every quarterly and annual filing starts from reconciled numbers.
• Monthly payroll tax deposits by the 15th, for a monthly depositor; a semiweekly depositor deposits within days of each payroll.
• Sales tax returns on each state’s schedule. Many monthly returns are due on the 20th, but states differ, so check each one.
• State withholding deposits on each state’s own schedule.

02January and February#

January 31 is the biggest federal date of the year, and in 2027 it’s a Sunday, so everything moves to Monday, February 1, 2027: Forms W-2 to employees and, with Form W-3, to the Social Security Administration; Form 1099-NEC to recipients and the IRS for each payee paid $2,000 or more by non-card methods in 2026; the fourth-quarter Form 941; Form 940 for federal unemployment tax (February 10 if all FUTA was deposited on time); and Form 945 if you backup-withheld from anyone. Recipient copies of Form 1099-MISC are due on the same date.

Paper Forms 1099-MISC are due to the IRS by the end of February (Monday, March 1, 2027, since February 28 is a Sunday); electronically filed ones by March 31. Most companies over the 10-return e-file threshold file electronically.

03March#

Delaware’s annual franchise tax report and payment for corporations are due March 1, which in 2027 is a Monday. The invoice Delaware sends is often calculated with the authorized shares method and can look alarming; recalculating with the assumed par value capital method, which needs year-end gross assets and issued shares, usually cuts it sharply. Late filing brings a penalty and monthly interest, and an unpaid company loses good standing. A Delaware LLC pays a flat annual tax by June 1 instead.

For S-corporations and partnerships (not a typical venture-backed C-corp), calendar-year returns, Forms 1120-S and 1065, are due March 15. Electronically filed Forms 1099-MISC and most other 1099s are due to the IRS by March 31.

04April#

The federal corporate income tax return, Form 1120, is due on the 15th day of the fourth month after year end: Thursday, April 15, 2027 for tax year 2026. Form 7004 gives an automatic six-month extension to October 15, but any tax owed is still due in April. The research credit is computed on Form 6765 with this return, and a qualified small business makes its election to apply up to $500,000 of it against payroll taxes here. The first 2027 estimated tax installment is due the same day.

The first-quarter Form 941 for 2027 is due April 30, a Friday.

05May through September#

The middle of the year is quieter at the federal level, which makes it the right time to clean up anything the year-end left behind:

• June 15 (a Tuesday in 2027): second corporate estimated tax installment.
• July 31: second-quarter Form 941. In 2027 July 31 is a Saturday, so it’s due Monday, August 2.
• July 31: Form 5500 for a calendar-year 401(k) plan, if you sponsor one and your provider doesn’t file it. Form 5558 extends it.
• September 15: third corporate estimated tax installment.

06October through December#

The year ends with filings for this year and preparation for the next:

• October 15: extended Form 1120 due, if you filed Form 7004.
• October 31: third-quarter Form 941. For 2026, October 31 is a Saturday, so it’s due Monday, November 2, 2026; in 2027 it’s a Sunday, so Monday, November 1, 2027.
• November and December: collect missing W-9s, confirm employee addresses and states, and report option exercises and fringe benefits to payroll before the last payroll of the year.
• December 15: fourth corporate estimated tax installment.
• December 31: year end. Count inventory if you hold any, and make sure the December close starts early, because the Delaware franchise tax needs year-end gross assets by March.

07When events set the date#

Some deadlines are triggered by something happening rather than by the calendar. One that no longer applies to most startups: FinCEN’s beneficial ownership (BOI) report. A 2025 interim rule exempted companies formed in the US, and a final rule effective August 14, 2026 made that permanent; only foreign companies registered to do business in a state still file. The event-driven ones that remain:

• An 83(b) election: within 30 days of a grant of restricted stock, with no extension and no fix for a missed one.
• A new employee in a new state: register for that state’s withholding and unemployment before the first payroll, and report the hire within 20 days, or sooner if the state requires.
• Crossing a state’s sales tax threshold: register before collecting, and start collecting on the state’s timeline.
• A priced round or a new option grant: usually a fresh 409A valuation, which will ask for current financial statements.