I am 66 and am not yet enrolled in Social Security or Medicare Part A or B. I am currently employed and have a HSA

Dupe HSA penalty with Medicare part A and Medicare Part A and a Health Savings Account (HSA) .

To be exact, it's not when you make the contributions, as such; it's how much you contribute compared to your limit, which depends on the number of months you were eligible i.e. NOT covered by Medicare. IF your semimonthly contributions were set at the yearly limit divided by 24 (or other number of pay periods), THEN the contributions for Jan.-June were within the limit and July-Sept. were excess. If the contributions were computed differently or varied, then it's the amount that matters not the timing. If you do indeed have excess contributions, you can avoid the penalty (and only pay tax on the ordinary income) by instructing the adminstrator to withdraw the excess contributions, with any earned income/gains (which should be small if you act promptly), BEFORE the return filing deadline (next April 15, or Oct. 15 if you get an extension, but you really should be able to get this done by April). Be clear this is a withdrawal/correction of excess contributions not a normal distribution; they need to know to report it differently. See Pub 969 at Excess contributions (not yet updated for 2020, but this part won't change).