Start Excel, ALT+F11 to open the Visual Basic Editor, Insert/Module then paste in the above code. Close the VBE. From your worksheet, Tools/Macro/Macros, highlight Count_Down_Timer and click the Run button.
VoG has given you a countdown stopwatch, the following code gives you a simple time elapsed stopwatch. The start time is placed in cell A1, when B1 is clicked the time elapsed since the start is displayed in the cell.
If you wish a second time based on the original start time then you will need to select a cell other than A1 or B1.
If you select A1 the sequence will be reset.
Sub Worksheet_SelectionChange(ByVal Target As Range)