The Expires property specifies the duration of time before a page that is cached on a browser expires. If the user returns to the same page before it expires, the cached version is displayed.
Use -1 to specify immediate expiration
When your .asp file calls Response.Expires, IIS creates an HTTP header, indicating the time on the server. If the system time on the client is earlier than the system time on the server (because of either the client or server having an inaccurate time setting, or time-zone differences), setting the parameter to 0 does not have the effect of expiring the page immediately. You can use the Response.ExpiresAbsolute property to immediately expire a page. In addition, you can use a negative number for the Expires property to expire the response immediately