In my time zone this works well:
set DateLabel=%date:~10,4%_%date:~7,2%_%date:~4,2%
ntbackup /.... /j %dateLabel%
You need to adjust x, y and z to work for your own time zone
by running this command from a Command Prompt:
echo DateLabel=%date:~x,4%_%date:~y,2%_%date:~z,2%