When is the deadline to start?

Two dates apply. The minimum required contribution for a calendar 2026 plan year is due September 15, 2027, eight and a half months after the plan year ends, under section 430(j)(1) of the tax code. The tax deduction is allowed if the business pays by its return due date, including extensions, under section 404(a)(6).

For a calendar-year S corporation or partnership on extension, both dates fall on September 15, 2027. For a sole proprietor or a C corporation on extension, the deduction date is October 15, 2027, a month after the funding date. A late minimum contribution draws an excise tax of 10 percent under section 4971, and 100 percent if the amount stays unpaid, even when the deduction itself is still on time.

An employer may adopt a plan after its tax year ends and treat the plan as adopted on the last day of that year, if the plan is adopted by the return due date, including extensions. Section 401(b)(2) sets that rule. Providers that require signing before the year ends are stating their own product rule, not the law.

A cash balance plan is a kind of defined benefit plan, so the rules in this answer apply to it.

The signing rule does not move the funding date. The minimum contribution for the year the plan counts for is still due eight and a half months after the plan year ends, so an owner who signs late in the window has less time to pay than the return date suggests.

For a single-employer plan, an unpaid minimum contribution draws an excise tax of 10 percent of the unpaid amount under section 4971(a)(1). If that tax is imposed and the amount is still unpaid when the taxable period closes, a second tax of 100 percent of the unpaid amount follows under section 4971(b).

The funding date and the deduction date are separate rules. A contribution paid after the funding date but before the deduction date can still be deducted and still owe the excise tax. The IRS states the rule on its own page for these plans: “An excise tax applies if the minimum contribution requirement is not satisfied.”

A bad year has its own set of steps, set out in order on the page about the risks of a plan.

Only in a year after the plan had a funding shortfall. Section 430(j)(3) requires quarterly installments when the plan had a shortfall for the preceding plan year, meaning its assets fell below the value of the benefits it had promised. A plan with no shortfall the year before pays its minimum once, by the funding date.

When installments apply and one is paid late, the interest charged on the late amount uses the plan’s own rate plus 5 percentage points. The enrolled actuary, the licensed actuary who certifies the plan’s funding each year, tells the employer when installments apply and how much each one is.

Section 412(d)(2) of the tax code lets the plan administrator elect to treat an amendment adopted no later than 2½ months after the plan year closes as made on the first day of that plan year, for the minimum funding rules. For a calendar 2026 plan year, that window closes Monday, March 15, 2027.

Such an amendment generally may not reduce benefits already earned. Whether a retroactive increase raises what the business may contribute and deduct for that year is a question for your enrolled actuary and counsel. A larger benefit still has to pass the coverage and nondiscrimination tests set out on the page about what employees get. The contribution still has to fit under the deduction ceiling in section 404(o), which an actuary figures, and the page about how much you can put away explains how that ceiling is set.

The questions to bring to your tax adviser about these dates are on the page about what to ask your CPA.