A Macro To Stop a Running Macro?

[ZAX]

Hello Everyone,
I Have The Following Macros To Start A Timer In Range A1 And they Work perfectly But The Problem Is That they NEVER Stop no matter what I do even if I removed The First code from the Model or if I pressed Esc; The Only way to stop it is to close the whole program 'Excel',The First Macro is Assigned to a Command Button On The Spreadsheet so I need a Macro Called "Stop_Timer" To Assign it to The Second Command Button To Stop The First Macro "Start_Macro"!
I Hope I'm Clear Enough And I Really Appreciate Your Help.
ZAX

Sub Start_Timer()
i = 0
Do
i = i + 1
Application.OnTime DateAdd("s", i, Now), "Add"
Loop
End Sub

Sub Add()
Range("a1").Value = Range("a1").Value + 1
End Sub

 

[Gary's Student]

Run this to stop the process:

Sub StopTimer()    
On Error Resume Next    
Application.OnTime EarliestTime:=RunWhen,Procedure:="Add", _
        Schedule:=False
End Sub

 

[ZAX]

Thanks For Your Help But Frankly, The Code Does Nothing At All. Mabe The Problem Is In My Code! If You Can Give Me Another One That Does The Same I'd Be Pleased.

 

[Gary's Student]

My error:

Sub StopTimer()
On Error Resume Next
Application.OnTime EarliestTime:=Now + TimeSerial(0, 0, 10), Procedure:="Add", _
    Schedule:=False
End Sub