California Startup Tax Deadlines: What Founders Need to Know
Most founders watch April 15 and nothing else. Depending on how your startup is set up, several other dates may apply across the year. Here's the federal, California, and Delaware calendar, and which dates tend to carry the steepest penalties.
WHY THIS MATTERS
Many California startups have federal and California filing obligations, and startups incorporated in Delaware or operating across state lines often have more. Which dates apply to you depends on your entity type and where you operate. A missed deadline can mean penalties, and in some cases can affect a credit you'd otherwise claim. The dates below assume a calendar-year filer; if your fiscal year differs, the relative timing holds but the exact dates shift.
The startup tax calendar
Gold dates are the ones founders most often miss or most regret missing.
NEXUS IN PLAIN TERMS
Apr 15
Q4 Estimated Tax Payment
Office, employees, or inventory in a state. Creates an obligation there.
Mar 1
Delaware Franchise Tax Report
Applies if your startup is incorporated in Delaware, as many are. Due to the Delaware Division of Corporations, and one of the most commonly missed filings because it's early and separate from anything you file in California.
Apr 15
Federal Corporate Return (Form 1120)
Your federal C-Corp return is due, or you file for a six-month extension to October 15. Filing on time is what preserves your R&D credit eligibility.
Apr 15
California $800 minimum franchise tax
The current-year minimum franchise tax is due to the FTB. See the franchise tax guide for who owes it.
Apr 15
R&D credit Check
For startups claiming the payroll-offset R&D credit, the timing of your return generally matters, and a late filing can affect eligibility. Confirm the current rules for your situation before you extend or delay. See R&D Tax Credits.
Jun 15
Q2 estimated tax payment
Second-quarter estimated payments to the IRS and California FTB.
Oct 15
Extended return deadline
If you filed a six-month extension in April, this is your final federal filing deadline. An extension to file is not an extension to pay.
WORTH UNDERSTANDING BEFORE YOU EXTEND
The R&D credit deadline isn't a separate date, it's tied to your return. Founders who extend or file late without understanding the interaction can affect a credit they'd otherwise be able to claim. Before you extend or delay, confirm how it affects your R&D position for your specific situation.
Foreign founders: an extra layer
Startups with foreign founders or foreign shareholders often have additional IRS reporting obligations, and these can carry steep penalties, commonly cited at $10,000 to $25,000 per form for late or missed filing depending on the form and circumstances. These forms generally track to your return deadline. It's a profile where a missed date can do real damage, and where getting a specialist involved before your first deadline tends to pay for itself. Verify which forms apply to your situation.
Need help staying on top of your startup's tax deadlines?
If you'd rather have a professional manage your filings and compliance, we can connect you with an accounting provider experienced with California startups.
What founders should do
→ Build the three-government calendar in month one. IRS, California FTB, Delaware. All three, not just April.
→ Star March 1. The Delaware report is the one people forget because it's early and it's a different state.
→ Don't extend casually if you're claiming R&D credits. Understand how the extension interacts with your credit before you file for one.
→ If you have foreign founders, get help before your first deadline. The penalties are automatic and large.
Deadlines are the cheapest tax problem to solve and the most expensive to ignore. Nothing here requires expertise to track. It just requires knowing the dates exist, which most first-time founders don't until they've already missed one.