Auto Log out

Hello ,
I use this to give access to my website :
<authentication mode="Forms">
<forms name="Login.aspx"/>
</authentication> And in the code I use :
FormsAuthentication.RedirectFromLoginPage(username,false);
 
Now , I seem to have 2 problems . First my program auto logs out me after a while . 2nd is that when I play with datetime setttings on the system it also disconnects me . Any ideas what I can do ? Thanks .

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