A full-year's-worth of contribution can be made under the last-month rule by anyone who is an HSA-eligible individual on December 1 of the year for which the contribution is being made. Keep in mind, though, if you make a contribution under the last-month rule and you fail to remain an HSA-eligible individual throughout the testing period, all of 2019 in this case, the amount contributed under the last-month rule that exceeds the amount that you were eligible to contribute were it not for the last-month rule is subject to a 10% additional tax. The amount subject to the 10% additional tax is not an excess contribution, so it is not permitted to be distributed as a return of excess contribution to avoid the 10% additional tax.
