Most penalties we see at the office are not about owing tax. They are about timing: an estimate paid late, an extension that was never filed, a K-1 that arrived after the owner's own return was due. Here is the calendar we work from, with what each date requires.

September 15, 2026

  • Third-quarter estimated tax for individuals, trusts, and calendar-year corporations. Federal Form 1040-ES and California Form 540-ES. Note that California front-loads its estimates: 30% in April, 40% in June, nothing in September, and 30% in January, so many Californians have no state payment due on this date.
  • Extended S corporation and partnership returns (Forms 1120-S and 1065) that were put on extension in March.

October 15, 2026

  • Extended individual returns, federal Form 1040 and California Form 540. This is the final deadline. There is no second extension.
  • Extended C corporation returns (Form 1120) for calendar-year corporations.
  • Last day to fund a SEP-IRA for 2025 if your return was extended.

January 15, 2027

  • Fourth-quarter estimated tax for 2026, federal and California. If you file your full return and pay the balance by January 31, you can skip this payment without penalty, though few people are ready that early.

January 31, 2027

  • W-2s to employees and 1099-NECs to contractors, and the copies to the IRS and Social Security Administration. Growers, packers, contractors, and harbor businesses with seasonal crews should have their vendor W-9s collected well before this.
  • Fourth-quarter payroll returns: federal Form 941 and California DE 9 / DE 9C, plus annual Form 940.

March 15, 2027

  • S corporation and partnership returns (Forms 1120-S, 1065, and California 100S / 565 / 568 for LLCs taxed as partnerships). K-1s go to owners on this date, which is why business returns are prepared before personal ones.
  • S corporation election (Form 2553) for a business that wants S status effective for all of 2027.

April 15, 2027

  • Individual returns, Form 1040 and California 540, or an extension request. An extension gives you until October 15 to file, but not to pay: any balance owed is still due April 15.
  • C corporation returns (Form 1120 and California Form 100).
  • First-quarter 2027 estimated tax, federal and California.
  • California LLC annual tax of $800 for the current year, due on the 15th day of the fourth month of the LLC's tax year.
  • Last day to make a 2026 IRA or HSA contribution.

Two rules that catch people every year

Weekends and holidays move the date forward. A deadline that lands on a Saturday, Sunday, or federal holiday moves to the next business day. Emancipation Day in Washington, D.C. occasionally shifts April 15 as well.

Disaster extensions are real but specific. When the IRS declares relief for Ventura County after a fire or flood, it publishes exact dates and applies only to taxpayers in the declared area. Do not assume; check the notice or ask us.

Clients of the office receive a reminder before each of these dates with a short list of what we need. If you are not a client and want the list anyway, call and we will send it. Staying ahead of these dates is part of what an accountant should be doing year-round.