@Mike9241 - Medicare has this '6 month' lookback provision. When you are accepted by Mediciare, you are ineligible for HSA contributions retroactive to 6 months before you get on Medicare, but not the months prior to turning 65 (not an issue here). OP;s maximum 2021 cotribution is based on 3/12 of whatever his 2021 maximum HSA contribution was calculated as, even if the contributions were made after March, 2021 (but before OP filed the 2021 tax return).
@Opus 17 has an excellent write-up in another post, which I can't put my hands on currently.
