users login/logout times on my site.

To solve this problem, you need a table with following structure -
UserID int
LogInTime datetime
LogOutTime datetime

Whenever user logs in, you need to enter a record in the table with LogInTime. When the user logs out, before logging him out, find the last record of the logged in user and update LogOutTime to current time.

However, this is not the best way as many users login to a website and close the browser after some time. If you want to implement some specific feature based on this, you must think an alternative solution as well.

  while login enter date in to database as logindate and onlogout update logoutdate and then calculate time according to it