If you are using ASP through IIS
On the Server side script you can use the
Session & Response Objects
The Session Object has a collection of objects/items, one of them (off the top of my head) actually has a property of LastUpdated
If you run in to problems with cache the following may help
Response.ExpiresAbsolute - Sets a date/time when the
cached page on the client will expire
Response.Expires - Sets how long (minutes) before the page expires on the client
Someone correct me If my memory fails
Hope this helps