Sub FlagByTime()
Dim aItem As Object
Dim strTime As String

Set mail = Application.ActiveExplorer.CurrentFolder
For Each aItem In mail.Items

'Check the message age
 If aItem.ReceivedTime > Date - 30 Then

strTime = TimeValue(aItem.ReceivedTime)

'Check the received time
  If strTime > #6:00:00 PM# Then
     aItem.Categories = "Nighttime"
     aItem.Save
 End If

 End If

Next aItem

 Set aItem = Nothing
End Sub