If you do not currently have a Worksheet_Change macro in your Data sheet, then copy the code below and paste it into the Data sheet code module. Access the code module by right clicking the sheet name tab, then click 'View Code' in the pop up menu. The code will run when you make a change to cell N5.
Private Sub Worksheet_Change(ByVal Target As Range)
If Not Intersect(Target, Range("N5")) Is Nothing Then
Sheets("Test").Range("E1") = Now
If you do have an existing Worksheet_Change macro in that sheet module, then copy and paste it in the reply box and we can combine the two so that you get the desired results. Excel will only allow one procedure per title per module and the change event has to have the title above to trigger.