newbie - problems with Select and timestamp(14) calculation

Hi I am trying to set-up a type of anti-flood block on one of my forms, the way I am doing this is that when the first submission is made I log various details into a user_log table including IP no and a Timestamp (14) column called log_time. Now if the button is clicked again the php script will run the following query: SELECT user_ip FROM clinic_log WHERE user_ip = '$pw_userIP' AND (((NOW()+0) - clinic_log.log_time ) < 300) What I am (trying) doing here is to check the users IP ($pw_userIP) against the user_ip column in the database and if it matches then check to see whether it has been there for less than 300 seconds. I have just spent the last couple of hours trying to get this working......it doesn't want to know. It keeps finding the matching IP and disregarding the time calculation...... Thanks for any help. Paul