I don't know if that last reply was directed at me or the OP, but I came up with code to tell when the page was last updated in the skin (I believe that comes from the last time the page settings were changed, but I am not positive)
The following code is something I just ran through a C# based skin of mine. I had hoped that the PortalSettings.ActiveTab.LastModifiedOnDate property could be used, but it doesn't look to be populated correctly, so I had to go and get a new tabinfo object based on the TabID from the ActiveTab object.
<%
var tc = new DotNetNuke.Entities.Tabs.TabController();
var ti = tc.GetTab(PortalSettings.ActiveTab.TabID, PortalSettings.PortalId, false);
Response.Write(ti.LastModifiedOnDate.ToString());
%>
As the reply from Richard states, this may or may not be what you are actually looking for.
This would tell you the last time a particular page was updated, after doing some quick testing that appears to be ONLY when the page settings are updated, not a module/content on the page.
So what you may actually want, is to have the audit information from a module being displayed.
Or come up with a custom module that looks at when any portion of the site changed, and display that date. That would require quite a bit of work though, work meaning looking at all pages, all modules, and figuring out what the most recent date was.