Creating different groups

Hi, I´m using Freeradius with MySQL authentication and accounting. Ok, if I want to allow access to a user, I add it to the radcheck table with UserName = "username" Attribute = "Crypt-Password" Op = "==" Value = "encryptedpassword" If I want to deny it I just add UserName = "username" Attribute = "Auth-Type" Op = "==" Value = "Reject" Now, my question is, which attribute, op and values should I use to allow access to a user only between certain hours? Which are the possible values that the field Attribute can have? Thanks for your comments, Edo