Does that 6900 then count as income for 2018 (requiring a corrected tax filing) or for 2019?
I'm not the OP, but I'm considering making a full 6900 HSA contribution for 2018 using the last month rule. If something unexpected happens in 2019 and I'm not on a HDHP the whole year, and that 6900 suddenly becomes 2018 income, that would suck - it would mean, among other things, falling over the ACA cliff. But if it's 2019 income, at least I have time to react. Do you know in which year I would need to claim it?
2019, based on the following from
https://www.irs.gov/pub/irs-pdf/p969.pdf:
Example 1.
Chris, age 53, becomes an eligible indi-
vidual on December 1, 2017. He has family HDHP cover-
age on that date. Under the last-month rule, he contrib-
utes $6,750 to his HSA.
Chris fails to be an eligible individual in June 2018. Be-
cause Chris didn’t remain an eligible individual during the
testing period (December 1, 2017 through December 31,
2018), he must include in his 2018 income the contribu-
tions made in 2017 that would not have been made ex-
cept for the last-month rule.