Critter#2 - thank you for your response; however I believe it is inaccurate. The W-2 is reported on a cash basis, regardless of what year the contribution applies to. When submitting the money to the HSA custodian, the year must be identified (current year or prior year), and the HSA custodian issues a Form 5498-SA that indicates the contributions made after the end of the year that apply to the prior year.
