There are many parts that you could modify and the scope of "Last site update" is quite big. You can create a user control and drop it on your master pate(template). Then if you want to get last update date for pages or content items you can subscribe for
events of the managers that are used for pages and content manipulations inside Global.asax - Application_Start. Then, check for command names like "CreatePage", "UpdatePage", "Publish", "PublishNew", "DeletePage", "DeleteLanguageVersion" or "CrateContent", "UpdateContent", "DeleteContent"
Telerik.Cms.CmsManager.Executed += new EventHandler<
Telerik.Cms.Engine.ContentManager.Executed += new EventHandler<
and get LastMofiedDate from the StagedVersion of ICmsPage or IContent object. You can persists the value in a text file or somewhere in a database table. Finally, your user control will read this date and display it on all pages.
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