In the US, the trade date, not the settlement date

In the US, the trade date, not the settlement date (T+2) locks in the loss or profit of a sale meaning 12/31 is the last day to lock in losses by selling for tax harvesting purposes. The exception is losses resulting from closing a short position, which go off of settlement date (closing a short for profit follows trade date norms for long sells). One can close a short position for a loss tomorrow 12/29 and utilize the tax loss for 2020. This is relatively moot given what appears to be a minuscule short position:

SHORT INTEREST

DATE SHORT INTEREST % CHANGE AVG. VOL
11/30/2020 320,552 31.97 1,719,768
11/13/2020 242,896 -44.84 2,370,006
10/30/2020 440,365 -2.99 705,668 No
10/15/2020 453,929 -15.88 1,569,436
09/30/2020 539,611 -2.22 3,554,359
09/15/2020 551,841 35.01 988,314 No
08/31/2020 408,751 95.03 1,230,457
08/14/2020 209,588 -7.22 1,211,252
07/31/2020 225,905 -20.65 1,297,239
07/15/2020 284,698 -12.27 555,231

In conjunction with the fact that you’d have to be pretty terrible at shorting to have a loss unrealized in GNBT to harvest.