The report is due "the last day of the seventh month after the plan year ends (July 31 for a calendar year plan)" (IRS, updated 2026-07-20). Form 5558 gives one extension of "up to 2 ½ months after the normal due date" (IRS, updated 2026-05-28), and the form itself caps the new date at the 15th day of the third month after the normal due date. For a calendar-year plan, that arithmetic gives October 15. A second route tied to the employer's tax extension cannot run "beyond a total of 9½ months beyond the close of the plan year" (2025 instructions).
The insurance details can be older than the plan year. Schedule A information "should pertain to the insurance contract or policy year ending with or within the plan year" (2025 instructions).
Worked example, using only those rules. A calendar-year plan's 2025 plan year ran January 1 to December 31, 2025. Its report was due July 31, 2026, and with an extension can arrive as late as October 15, 2026. So on 2026-10-06, an extended report for that year can still be on time. When it arrives, a policy year that matches the plan year describes coverage that started on January 1, 2025, about 21 months earlier. If the insurance contract runs July 1 to June 30, the 2025 report carries the contract year that began July 1, 2024.
The full picture runs later still: DOL's 2026 Report to Congress covers 2023, "the latest year for which complete data are available." A policy-year end date read off an old report is a date, not a decision. The X-dates and renewal dates guide covers what a date can and cannot tell you.