How to maintain Login and Logout time in Asp.net ?

How can i maintain the Login and Logout time in Asp.net ?

I have store Login and Logout time in the Database..

once user is logged in, we will capture the login data..... but the question is logout time ?

And i have tried to use Session_End Event in global.asa file... 

thn hw can i maintain the logout time.........

For example (Facebook,Gmail) how they will maintain the user weather the user are in offline r online ?

Same like that i was implementing chatting application, can u guide means to achieve this solution

Thank you....

prasad