Or you could do it this way:

UPDATE w3t_Users
SET U_Registered=UNIX_TIMESTAMP('2002-11-27 23:35:00')
WHERE U_Username='whomever'

UPDATE w3t_Users
SET U_Registered=UNIX_TIMESTAMP('2002-11-27')
WHERE U_Username='whomever'

The second example sets the time of day to 00:00:00.