When the SlidingExpiration is set to true, the time interval during which the authentication cookie is valid is reset to the expiration Timeout property value. This happens if the user browses after half of the timeout has expired. For example, if you set an expiration of 20 minutes by using sliding expiration, a user can visit the site at 2:00 PM and receive a cookie that is set to expire at 2:20 PM. The expiration is only updated if the user visits the site after 2:10 PM. If the user visits the site at 2:09 PM, the cookie is not updated because half of the expiration time has not passed. If the user then waits 12 minutes, visiting the site at 2:21 PM, the cookie will be expired.
Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)