online/offline script help!!

Yo! I'm working on a script wich will keep track of user online or offline. = When a user logs into a site, the script will register the user with an = unique ID and the time when the user logged in, in a database table. On = further requests from the user on any page of the site a script will = query a database for the users unique id and time. If the ID exists and = it is still valid, the script will update the time column with the = current time. When a user has been inactive (not have had any requests = that have updated the time coumn) the user is seen as offline. My = problem is the time. In the database table the column is of type = DATETIME and I want to know how to figure out if the user is online or = not. The original idea I had is: If table_time is x min older than NOW() = then user =3D offline How can I do this? Answers much appreciated /Broder