I guess you want something like this:
Option Explicit
Dim stopped As Boolean
Sub TimerStart()
Dim start As Integer
stopped = False
start = CInt(Timer)
With Range("B1").FormatConditions
.Delete
.Add Type:=xlCellValue, Operator:=xlGreaterEqual, Formula1:="120"
.Parent.FormatConditions(1).Interior.ColorIndex = 3
End With
While Not stopped
If start <> CInt(Timer) Then Range("B1") = CInt(Timer) - start
DoEvents
Wend
End Sub
Sub TimerStop()
stopped = True
End Sub
Use TimerStart as the macro for your first button, and TimerStop as the macro for the second button. If you're using OLE button object (rather than form objects), use the code in the CommandButtonX_Click events.