A SEP IRA employer contribution for a tax year is often allowed after December 31—generally until the business’s tax filing deadline including extensions. That calendar is different from a W-2 employee elective deferral deadline inside a 401(k) or solo 401(k), and different from the annual dollar / percentage limit itself. Custodians such as Fidelity, Schwab, Vanguard, and E*TRADE need the deposit labeled for the correct tax year.
Who a SEP fits and how limits scale with net self-employment income: SEP IRA basics for side income. Solo 401(k) deferral timing is a different machine: Solo 401(k) basics. HSA prior-year deposits are a cousin product with its own rules: HSA contribution deadline basics.
Three clocks people mix up
| Clock | What it controls | Typical cue (verify the year’s IRS pubs) |
|---|---|---|
| Annual SEP limit | How much employer SEP money you may add for that tax year | Percentage of eligible compensation + IRS dollar cap |
| SEP contribution deadline | Last day you can still fund a prior tax year’s SEP | Often the business return due date with valid extension |
| Employee deferral deadline (401(k) / solo 401(k)) | Last day elective deferrals for a year must be withheld / deposited under plan rules | Usually tied to the calendar year / payroll—not the SEP extension window |
A filing extension that stretches your Form 1040 or business return can stretch SEP employer funding in many years. It does not magically reopen a missed employee elective deferral window for a 401(k). Confirm IRS Publication 560 / Form 5305-SEP instructions for the year you are funding—do not copy a blog date onto every plan type.
Tax-season habits: Filing taxes for beginners. Where SEP money sits in a beginner portfolio: Investing basics for beginners.
Practical funding steps
- Estimate SEP-eligible compensation from net self-employment profit (after the usual SE-tax adjustment)—not gross 1099 deposits (SEP side-income basics).
- Decide whether you need an extension mainly to finish books and still fund the SEP, or whether cash is ready by the unextended deadline.
- Tell the custodian the contribution is for tax year 20XX when you deposit between January 1 and the applicable deadline.
- Keep the wire/ACH confirmation and the adoption agreement (prototype or Form 5305-SEP) with your tax file.
- If you also run a solo 401(k) or SIMPLE IRA, map each plan’s deadline on one page so payroll deferrals are not confused with SEP employer deposits.
Worked example: funding a prior-year SEP in September
Jordan is a sole-prop designer. For tax year 2025, Schedule C profit after expenses is about $48,000. After the SE-tax adjustment, SEP-eligible compensation supports roughly a mid–five-figure employer contribution at the usual percentage limit (illustrative—run IRS worksheets). Jordan files an extension and funds a $9,000 SEP deposit at Schwab on September 12, 2026, selecting tax year 2025 in the contribution flow.
If Jordan had left the deposit on the default 2026 tax year, the 2025 return would under-report the SEP deduction and 2026 might later risk an excess or wrong-year mismatch. Jordan’s day-job 401(k) deferrals for 2025 were already locked when payroll closed—no September “catch-up deferral” through the SEP extension window.
Common pitfalls
- Treating the SEP extension window like a 401(k) employee deferral deadline.
- Funding on gross revenue instead of eligible compensation.
- Depositing after the unextended deadline without a valid extension when the year’s rules required one.
- Mixing SIMPLE IRA employee deferral calendars with SEP employer timing.
- Ignoring employees: if eligible staff exist, the same percentage usually applies—budget before you fund yourself only.
Checklist
- Separate annual limit, SEP funding deadline, and any 401(k) deferral deadline on one note.
- Confirm whether an extension is filed before you rely on a late SEP deposit date.
- Label custodian deposits with the correct tax year.
- Coordinate cash with quarterly estimates so the SEP does not crowd out taxes owed (Quarterly estimated taxes).
- Revisit the calendar when you hire employees or switch to a solo 401(k).
- Verify the year’s IRS Publication 560 / custodian prior-year contribution screens before you wire.
SIMPLE IRA employee remittance vs employer match calendars: SIMPLE IRA contribution deadline basics.
Solo 401(k) elective deferral deadlines vs employer profit-sharing dates: Solo 401(k) employee deferral deadline basics.
Educational only. Not tax, legal, or investment advice. SEP deadlines, limits, and extension interactions change by tax year; verify IRS.gov publications, Form 5305-SEP instructions, and your custodian’s prior-year contribution procedures before you deposit.