You'd have to put a new date:
UPDATE w3t_Posts
SET B_Posted = xxxxxxxxxxx
WHERE B_Number = yyyyy
Where I have xxxx - that's the date - in epoch time. Epoch time is the number of seconds from January 1 1970. I've got a time converter here:
http://www.joshuapettit.com/time.phpWhere i have yyyy that's the post number.