This post covers filing deadlines and general compliance timing. It is not tax advice. Consult your CPA or tax advisor for guidance specific to your situation.

If you incorporated in Delaware and operate in California, you are dealing with three taxing jurisdictions at the federal level, two at the state level, and potentially a fourth if you are based in San Francisco. Each has its own forms, its own deadlines, and its own penalties for getting it wrong.

Most founders learn about these deadlines reactively. A late-filing notice shows up. Their accountant flags a missed estimated payment. Delaware suspends their entity two weeks before a fundraise because nobody paid the franchise tax. This post is the calendar we wish every founder had pinned to their wall on January 1.

We have organized everything chronologically. Bookmark this, share it with your ops lead, and set calendar reminders now.

January: W–2s, 1099s, and Q4 Cleanup

January 15, 2027 (for tax year 2026) is when your fourth-quarter federal estimated tax payment is due. We mention it here because if you are reading this mid-year, you need to know it is coming. For the current year, your Q4 estimated payment for 2025 was due January 15, 2026.

January 31 is when quarterly payroll tax filings wrap up for the prior quarter. California’s DE 9 and DE 9C (Quarterly Contribution Return and Report of Wages) for Q4 are due January 31, 2027. If January 31 falls on a weekend, the deadline shifts to the next business day. Handled by your payroll provider (Gusto, Rippling)

This is also the month to finalize your W–2s and 1099s. Federal law requires you to furnish Form W–2 to every employee and file with the Social Security Administration by January 31. For 2026, that date falls on a Saturday, so the effective deadline is February 2, 2026. The same applies to Form 1099-NEC for independent contractors.

If you paid contractors $600 or more during the year, you need a 1099-NEC filed by that date. Miss it and you are looking at IRS penalties starting at $60 per form, scaling up to $310 per form if you file after August 1.

February: San Francisco Business Tax

If your startup is based in San Francisco, March 2, 2026 is the deadline for your Annual Business Registration and Tax Form. (The statutory date is February 28, which falls on a Saturday in 2026.) This single filing now covers your Business Registration Renewal, Gross Receipts Tax, Homelessness Gross Receipts Tax, Commercial Rents Tax, and Overpaid Executive Gross Receipts Tax, all consolidated under Proposition M, which took effect January 1, 2025.

Here is the good news for smaller startups: Prop M raised the Small Business Exemption threshold from $2.25 million to $5 million in combined San Francisco gross receipts. If you are under that number, you are exempt from the Gross Receipts Tax and Overpaid Executive Tax. You still need to file the form and renew your registration. Skipping the filing because you think you are exempt is a common mistake, and it comes with a $100+ penalty.

For companies above $25 million in San Francisco gross receipts, the Homelessness Gross Receipts Tax now applies. That threshold dropped from $50 million under Prop M.

If you need more time, you can request an extension to November 30, 2026, but you must submit the extension request and make any required payment by March 2.

March: Delaware Franchise Tax and Pass-Through Returns

March 1, 2026 is the Delaware Annual Franchise Tax and Annual Report deadline. Every corporation incorporated in Delaware owes this, regardless of whether the company does any business in Delaware or earns revenue there. There is no extension available for this filing.

This is the deadline that catches startups off guard during fundraising. If you miss it, Delaware charges a $200 penalty plus 1.5% monthly interest on the tax and penalty owed. Worse, your entity can lose its good standing, which will stall any due diligence process.

Two things every founder should know about the calculation. Delaware defaults to the Authorized Shares Method, which bases your tax on the total number of shares authorized in your Certificate of Incorporation. If you have authorized 10 million shares (common for startups that have raised venture capital), the default calculation can produce a tax bill north of $80,000. The alternative is the Assumed Par Value Capital Method, which factors in your actual issued shares and total gross assets. For most startups, this method brings the bill down to the $400 minimum. You have to actively select this method when filing. If you just pay the number Delaware’s system spits out, you are almost certainly overpaying.

March 16, 2026 is the federal filing deadline for S corporations (Form 1120-S) and partnerships (Form 1065). The standard date is March 15, which falls on a Sunday in 2026. California’s corresponding returns follow the same deadline. If you need more time, file Form 7004 by March 16 to get an automatic extension to September 15, 2026. The extension gives you time to file, not time to pay. Any tax owed is still due March 16.

California’s DE 9 and DE 9C for Q1 payroll taxes come due at the end of the following month: April 30, 2026.

April: C-Corp Returns, Estimated Taxes, and the $800 Franchise Tax

April 15, 2026 is the big one for C corporations. This is the federal filing deadline for Form 1120 and the California filing deadline for Form 100.

It is also the date your first quarterly estimated tax payment is due, both federally (Form 1120-W) and to California (Form 100-ES). Federal estimated tax payments become mandatory once your expected tax liability hits $500. California’s threshold is $800 (which is also the minimum franchise tax).

Speaking of the $800: every corporation and LLC registered in California owes an $800 minimum franchise tax annually, due by the 15th day of the fourth month of the taxable year. For calendar-year entities, that is April 15. This is separate from your income tax return. Even if your startup has zero revenue, you owe the $800. The one exception: California now exempts newly formed or newly registered entities from the $800 minimum in their first taxable year. This exemption, introduced in 2024, is still active for entities formed in 2026.

If you need a federal extension, file Form 7004 by April 15 for an automatic extension to October 15, 2026. California follows the federal extension automatically; you do not need to file a separate state extension if you have a valid federal one. But again, the payment is still due April 15.

June: Second Estimated Tax Payment

June 15, 2026 is the due date for your second quarterly estimated tax payment to the California FTB (Form 100-ES). The federal second quarter payment is due June 16, 2026 (the 15th falls on a Monday in 2026, but the IRS lists this as June 16 since June 15 is not a holiday; double-check the IRS calendar for the exact date as these shift).

These estimated payments are not optional if you have a tax liability. Underpayment penalties accrue when you miss them, and they are calculated from the original due date, not from when you eventually file your return.

California’s DE 9 and DE 9C for Q2 payroll taxes are due July 31, 2026.

September: Extended Pass-Through Returns and Third Estimated Payment

September 15, 2026 is the extended filing deadline for S corporations and partnerships that filed Form 7004 back in March. This applies to both federal (Forms 1120-S and 1065) and California returns.

It is also the due date for the third quarterly estimated tax payment, both federally and to California.

California’s DE 9 and DE 9C for Q3 payroll taxes are due October 31, 2026.

October: Extended C-Corp Returns

October 15, 2026 is the extended federal filing deadline for C corporations that requested more time back in April.

December: Fourth California Estimated Payment

December 15, 2026 is the due date for the fourth quarterly California estimated tax payment (Form 100-ES). Note that the federal fourth quarter payment is not due until January 15, 2027. This mismatch trips people up every year. California wants its fourth installment a month earlier than the IRS does.

The Full Calendar at a Glance

January 15 – Q4 federal estimated tax payment (for prior year)

February 2 – W–2s and 1099-NECs due to recipients and filed with SSA/IRS

March 1 – Delaware Annual Franchise Tax and Annual Report

March 2 – San Francisco Annual Business Registration and Tax Form (if applicable)

March 16 – S-corp and partnership federal and CA returns (or extension)

April 15 – C-corp federal and CA returns (or extension); Q1 estimated tax payments (federal and CA); California $800 minimum franchise tax

April 30 – CA EDD payroll taxes (DE 9/DE 9C) for Q1

June 15/16 – Q2 estimated tax payments (CA and federal)

July 31 – CA EDD payroll taxes for Q2

September 15 – Extended S-corp/partnership returns; Q3 estimated tax payments

October 15 – Extended C-corp federal return

October 31 – CA EDD payroll taxes for Q3

December 15 – Q4 California estimated tax payment

January 15 (next year) – Q4 federal estimated tax payment

January 31 (next year) – CA EDD payroll taxes for Q4

What Actually Goes Wrong

The deadlines themselves are not complicated once you see them laid out. The problems come from a few predictable places.

First, Delaware franchise tax gets forgotten because the company does not operate in Delaware. There is no revenue there, no employees there, and no reason to think about the state until March 1 passes and you are out of good standing.

Second, the Authorized Shares Method is the default calculation for Delaware franchise tax, and it wildly overstates what most startups owe. Every year, we see founders who paid $50,000 or more when their actual liability was $400. If your accountant is not switching you to the Assumed Par Value Capital Method, ask why.

Third, California estimated tax payments have a different Q4 date than the federal ones. If you are setting reminders based on the IRS schedule, you will miss the December 15 California payment.

Fourth, San Francisco’s consolidated tax filing under Prop M is new. The first filing under the new system covered tax year 2025, due March 2, 2026. The categories changed, the apportionment formula changed (from payroll-weighted to 75% sales/25% payroll), and the exemption thresholds changed. If you are filing based on what you did in 2024, you are filing wrong.

Fifth, W–2s and 1099s are due earlier than most founders expect. The January 31 deadline (February 2 in 2026) is firm, and the IRS has been enforcing late-filing penalties more aggressively in recent years.

Set It and Forget It (Almost)

The best thing you can do is put these dates into a shared calendar the moment your fiscal year starts. Not a spreadsheet someone has to remember to check. A calendar with alerts, assigned to whoever owns finance at your company.

If you have a bookkeeper or accountant, make sure they are tracking all three jurisdictions, not just federal. We have seen plenty of startups where the tax preparer handles the 1120 but nobody is watching the Delaware franchise tax or the San Francisco filing. That gap is how penalties happen.

If you want to talk through how this applies to your company, book a call.

Anelya Grant is the founder of AG Accounting Inc. (anelya.net), an accounting firm serving tech startups and healthcare organizations. She is also co-founder of JustPaid.ai, an AI-powered billing and contract-to-cash platform for growing companies.