if she is cover by a HDHP on 12/1/2023 she can make a full year's contribution to her HSA. The warning is that there will be a penalty in 2024 if she drops HDHP coverage before 12/31/2024. This is dome through the use of the last month rule.
Last-month rule. If you are an eligible
individual on the first day of the last
month of your tax year (December 1 for
most taxpayers), you are considered to
be an eligible individual for the entire
year, so long as you remain an eligible
individual during the testing period as
discussed below.
Testing period. You must remain
an eligible individual during the testing
period in order to take advantage of the
last-month rule. The testing period
begins with the last month of your tax
year and ends on the last day of the
12th month following that month (for
example, December 1, 2023 –
December 31, 2024). If you fail to
remain an eligible individual during this
period, other than because of death or
becoming disabled, you will have to
include in income the total contributions
made that would not have been made
except for the last-month rule. You
include this amount in income in the
year in which you fail to be an eligible
individual. This amount is also subject to
a 10% additional tax.
