|You didn't supply any information about the how/where the values are placed in the Excel spreadsheet, so it's hard to be very specific.|
One possible solution is a WorksheetChange macro that monitors the spreadsheet and enters a time when a specific cell changes.
Of course, not knowing how your 3rd party hardware/software puts the data in the spreadsheet, I can't say how well this would work.
For example, let's say the software put the values in Column A. Assuming that Excel/VBA can recognize the entry of the data, this might work to place a time stamp in Column B on the same row:
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Column = 1 Then _
Target.Offset(0, 1) = Now