Can't Stop Macro Running At Set Time

I wanted to run a macro at 08:00 each day. I tested the macro would auto start and run, and set the test to start at 13:00.

In the Private module of the Workbook I put

Private Sub Workbook_Open()
Application .OnTime TimeValue("08:00:00"), "MyMacro"
End Sub

and in a standard module

Sub TestTimeMacro()
Application.OnTime TimeValue("08:00:00"), "MyMacro"
End Sub

Initially I had the TimeValue set as 13:00:00. The test worked OK so I changed the timevalue to 08:00:00.

The macro now runs at 08:00 and 13:00.

How do I stop the 13:00 event?