I've never seen this one before... a 2020 1099-B with gross proceeds and no cost basis, and a 2019 "date sold." I see on his 2019 return that the sale was reported (at a loss), and client doesn't recall any 2020 activity for the transaction.

So if the broker made an error on the 2019 1099-B, would they correct it with a 2020 1099-B? That seems wrong; but that's the only explanation I can come up with for why I've got this form in my 2020 documents.

Do I need to report it on the 2020 return?

If so, leave the date sold out or make up a 2020 date?