Excel 2007 : Automatic update NOW function every second

I have made a service advisor monitoring sheet as attached. In column M I need to monitor the time remaining to deliver vehicle.Which will be Date & time promised in column K - Current date & time. I need this reaining time to update every second. I Got some Macro to do this but i am getting some error while closing this workbook as the clocck is still running. I need to start this macro automatically and stop before closing the workbook. Kindly advise to correct this macro code.Regards.