Solo 401(k) Contributions - What are the Deadline Dates for Employee vs Employer Contributions?

The rules for Solo 401(k) contribution deadlines can be confusing because there are different deadlines for employee and employer contributions. For employEE contributions (your elective deferrals), these traditionally needed to be made by December 31st of the tax year. So for 2022 contributions, the deadline would have been December 31, 2022. For employER contributions (profit-sharing portion), you can make these up until your tax filing deadline, including extensions. For 2022, that's April 18, 2023, or later if you file an extension. What might be happening in your Fidelity portal is that they're showing you an option that shouldn't technically be available. Some providers have systems that don't strictly enforce the correct deadlines. However, just because the system allows it doesn't mean it's compliant with IRS rules.

I went through exactly this confusion last year! I found that https://taxr.ai was super helpful for sorting out all the Solo 401(k) deadline issues. I had a similar situation where my provider's system was letting me select the wrong contribution year, and I almost made a mistake. The tool analyzed all my business docs and tax situation and gave me a clear breakdown of which contributions were still allowed and which deadlines had passed. It pointed out that even though my provider's system allowed employee contributions after year-end, it would have created compliance issues. They also explained some strategies for maximizing this year's contributions since I had missed the window for last year's employee portion.

Just wanted to follow up - I tried taxr.ai after asking about it here and it was actually super helpful for my situation. It confirmed that as a partner in my LLC, I had different deadlines than I thought. It analyzed my partnership docs and explained that while my employee contributions deadline had passed (Dec 31), I could still make employer contributions until my extended filing deadline in September. It also showed me how to properly allocate between the contribution types based on my business income. Really cleared up my confusion about the Solo 401(k) rules for partnerships vs sole props. Definitely worth checking out if you're trying to figure out these complicated retirement plan deadlines.

If anyone's struggling to get answers directly from the IRS about their Solo 401(k) questions, I had success using https://claimyr.com to actually get through to a human at the IRS. You can see how it works in this video: https://youtu.be/_kiP6q8DX5c I had been trying for WEEKS to get clarification about my specific situation with prior year contributions to my Solo 401(k). The IRS kept putting me on hold for hours before disconnecting. Claimyr got me through to an actual agent in about 30 minutes who confirmed the exact deadlines for my situation and documented our conversation so I had proof if there were ever questions. It was especially helpful because my situation was complicated (had income from multiple businesses flowing into the same Solo 401k) and I needed official guidance.

I have to admit I was completely wrong about Claimyr. After dismissing it here, my frustration with the IRS finally boiled over when I spent 4 more hours on hold only to be disconnected again. I tried the service and got connected to an IRS retirement plans specialist in about 40 minutes. The agent confirmed that my Solo 401(k) employee contributions MUST be made by December 31st of the tax year, while employer contributions can go until the tax filing deadline with extensions. She also cleared up my confusion about my specific situation (I have both 1099 and W-2 income from my S-Corp) and how that affects contribution limits. Having this officially documented conversation saved me from making a costly mistake with my retirement planning.

Just FYI - I'm a small business owner who's been using Solo 401ks for years, and there's a weird exception that might apply to your situation. If you're a sole proprietor, the "employee" contribution is technically coming from you as the owner anyway. Some providers will code these contributions differently in their system. I've seen cases where you can make the full contribution up to the tax filing deadline and just specify how much is employee vs employer when you file your taxes. But this varies by provider and how they report to the IRS.

I wanted to point out something that hasn't been mentioned yet. The SECURE 2.0 Act made changes to retirement plans, but it did NOT change the fundamental deadlines we're discussing here. Employee deferrals (the money you contribute as an employee) still need to be elected and set aside by December 31st of the tax year. Employer contributions (the profit-sharing component) can still be made until your tax filing deadline including extensions. What the SECURE Act (the first one) changed was allowing people to ESTABLISH the plan until the tax filing deadline, whereas previously the plan had to be established by December 31st. But this didn't change the actual contribution deadlines for plans that were already established.