That's still back-dating the check, which is a no-no.

The shareholder knows what their December health insurance premium will be at least a month in advance of the payment date. I could tell you today what my December 31st premium payment will be, because I've already signed the renewal paperwork for 2024. Either have the S-corp pay the December payment directly, have the shareholder actually reimbursed in December, or get the deduction in 2024.