General :: Users Logging In And Out

I am trying to implement a username and password system on my DB. What I have done so far is created a table with username, password, permission level and datestamp. So when the db is opened, the form pops up and someone has to sign in. When they hit the 'login' button, there is a field that will populate the time in which they signed in. This will also allow me to see who is logged in. When they are finished, they will end up on the login screen where there is a close button. When they hit the close button, the field with the datestamp will be cleared. This will allow me to know that the user is no longer on the db. Here comes the question, if a user just closes access without hitting my 'close' button, my datestamp field will not be cleared. I have tried using the "onclose" event, but of course i used 'me.datestamp = "" but the form is already closed, so it says 'can not change the value of this field.' How can i clear the value of the datestamp field for a specific user when a user closes the program the "improper" way.