Pension plan deadline calendar.

Owners get into trouble by treating “year-end” as the only date. For a calendar-year defined benefit or cash balance plan, at least three clocks run.

• Minimum funding is generally due 8½ months after the plan year ends — September 15 for a calendar year. Missing it can mean a 10 percent excise tax on Form 5330. This is not the same as the deduction deadline.
• The deduction for an employer contribution is often available if the deposit is made by the tax-return due date, including extensions, under section 404(a)(6). Your CPA confirms that. A September 15 deposit can be earlier than you need for the deduction, or later than you should wait if funding is already late.
• Form 5500 is generally due the last day of the seventh month after year-end — July 31 — and Form 5558 can extend that to October 15. Filing is electronic on EFAST2. See the IRS Form 5500 corner and the Department of Labor’s filing page.

A new plan can often be adopted as late as the filing deadline, including extensions. Do not start the design then. Census, documents, and an actuary take weeks. Employee 401(k) deferrals generally cannot be created retroactively through payroll. The dated essays are in the briefing. If you are inside a window now, call before you guess.