The HSA proration widget: your last year, in twelfths

The three facts the arithmetic stands on

One: any Medicare — even premium-free Part A — ends contribution eligibility, month by month. Two: Part A filed for after 65 is backdated up to six months (never before the 65th-birthday month), so the month to enter above is the effective month after backdating, not the month you signed anything. Three: the $1,000 catch-up prorates by the same twelfths — a July Medicare start allows 6/12 of limit-plus-catch-up, which for self-only coverage is exactly the $2,700 worked through on the HSA guide.

Worked examples, both directions

• Retiring mid-year: family coverage, 58 years old, Medicare effective September 1 → 8 eligible months → ($8,750 + $1,000) × 8/12 = $6,500. Contributions above it are excess — withdrawable with earnings before the tax deadline to avoid the excise tax.
• The backdating ambush: same person files for Part A in September; coverage backdates to March 1 → only 2 eligible months → $1,625. Six months of "normal" contributions just became excess retroactively — the exact failure the six-months-early stop rule prevents.
• Delaying everything: still working at a 20+-employee firm, no Medicare, no Social Security → full limit, every year, with the 8-month SEP waiting at the exit.

Two veteran-specific footnotes the widget flags but can't compute: recent non-preventive VA care for a non-service-connected condition pauses HSA eligibility for three months (service-connected, preventive, dental, and vision care don't), and once contributions end, the account's second life begins — tax-free spending on Part B premiums, IRMAA included, plus VA copays, forever.