You can increase the timeout, but you shouldn't have to.
To increase it, see here: http://msdn.microsoft.com/en-us/library/h6bb9cz9(vs.80).aspx
But you should cope with the session timing out gracefully, by checking, or by using a try...catch block.
Oh, and by the way, never read the date and time twice - read it once, store it in a variable, and use it twice instead:
DateTime now = System.DateTime.Now;
lblDate.Text = now.ToLongDateString();
lblTime.Text = now.ToShortTimeString();
lblWelcomeMSG.Text = Session["UserName"].ToString();
catch (Exception ex)
If you read twice, you can get different days, months, or even years and this can cause subtle bugs which are hard to track down. In this case, it just presents a bad date and time combo to the user, but if you use similar code for databases you can truly mess up your data!