Because you had Self-only coverage on December 1, 2019, then your annual HSA contribution limit for 2019 is $3,500, thanks to the last-month rule. That's $4,500 if you are 55 or older.
The last-month rule lets you use the full annual limit if you were covered in the last month of the year.
The catch is that you have to stay under HDHP coverage throughout the testing period, which is essentially all of 2020. If you don't, then you will be penalized.
If you think you are likely to go off of HDHP coverage (or get conflicting coverage like Medicare) during 2020, then you should limit your actual 2019 contributions to 1/3rd of the normal annual limit (i.e. $1,500 for $4,500, or $1.167 for $3,500). Then, even if you have to admit to a "lapse" in HDHP coverage in 2020, it will be OK because when you enter that you were covered for 4 months, TurboTax will determine that your contributions were less than or equal to the prorated limit.
Your husband's limit will be $2,626 (assuming that he had only HDHP insurance on July 1 and no conflicting coverage).
***NOTE*** If you make contributions now for 2019, you MUST make sure that you tell the HSA custodian that the contributions are for 2019. Otherwise they will use the default year of 2020, and that will really foul up the paperwork (and what gets reported to the IRS).
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