I actually made a small module for this once.
It checked:
- tab/page last updated:
Dim objTabController As New TabController
Dim objTabInfo As TabInfo = objTabController.GetTab(PortalSettings.ActiveTab.TabID, PortalSettings.PortalId, True)
dates.Add(objTabInfo.LastModifiedonDate)

then it checked all the modules on the page:

Dim objModuleController As New ModuleController
Dim lstModuleInfo As Dictionary(Of Integer, ModuleInfo)
lstModuleInfo = objModuleController.GetTabModules(PortalSettings.ActiveTab.TabID)
Dim a As KeyValuePair(Of Integer, ModuleInfo)
Dim objModuleInfo As ModuleInfo
For Each a In lstModuleInfo
objModuleInfo = a.Value
If (objModuleInfo.IsDeleted = False) And (objModuleInfo.StartDate <= Now.Date) And (objModuleInfo.EndDate <= Now.Date) And (objModuleInfo.ModuleID <> Me.ModuleId) Then
objModuleController.GetModuleSettings(objModuleInfo.ModuleID)
'give the lastmodified date of the Module (not the TabModule I guess a bug in DNN)
dates.Add(objModuleInfo.LastModifiedOnDate)
End If
Next

And after that it checked the contents of some modules:

If lstModuleNames.Contains("DNN_HTML") Then
GetHTML()
End If

If lstModuleNames.Contains("DNN_Links") Then
GetLinks()
End If

If lstModuleNames.Contains("DNN_FAQs") Then
GetFAQs()
End If

If lstModuleNames.Contains("DNN_Announcements") Then
GetAnnouncements()
End If

etc...


So I made some stored proc. to get the last modified date for each type of module:

Public Sub GetHTML()
Dim HTMLdate As Date = Null.NullDate

HTMLdate = DotNetNuke.Data.DataProvider.Instance().ExecuteScalar("Captise_LastModified_TextHTML", TabId)

If HTMLdate <> Null.NullDate Then
dates.Add(HTMLdate)
End If
End Sub

And at last I got the newest date from the dates list:

I'm intersted to your article. I use DotNetNuke 4.9 and I want to display a Last Modified Date and User on the bottom of any page. Can you to share your solution with me or, please, can you provide me more informations? Thank you