Do you have an order by clause in the SQL:
WHERE User_JoinDate >= '2010-12-02' AND User_JoinDate <= '2010-12-08' AND User_AccountType = 'free' ORDER BY User_JoinDate ASC
you can use the MySQL CURDATE() command to return the currant date and use DATE_SUB to subtract 7 days
CURDATE()
DATE_SUB(CURDATE(),INTERVAL 7 DAY)
or
WHERE User_JoinDate >= 'DATE_SUB(CURDATE(),INTERVAL 7 DAY)' AND User_JoinDate <= 'CURDATE()' AND User_AccountType = 'free' ORDER BY User_JoinDate ASC
see the mysql page for more details on date and time functions:
date-and-time-functions.html