Each brokerage might be slightly different. Can't confirm on that, but I use TDA and ETrade online individual brokerage accounts. I just e-chatted with TD and Etrade and they both confirmed that as long as the buy/sell took place in 2019, even on the last day: 12/31, that it would be on the 2019 1099.

Strangely, I've always been successful at picking up dumps to the bid in the last few trading days of the year.