Problem adding more countdown timers

Hi I work in a school for the exams officer and am trying to create a spreadsheet that can be displayed at the front of the exam hall to advise the students of the start and finish times for the exams as well as having a countdown timer to show them how long they have left.

The spreadsheet should have the ability to have up to 5 separate examinations with countdown timers running simultaneously. I have created buttons and code for the first exam and could use this countdown timer for any other exams starting at the same time. However, the problem is that the exams do not all start at the same time - for example some of the languages papers are short and run consecutively during a single exam session but need to have individual timers. Also as soon as I type anything else the counter stops which I also do not want to happen.

I have attached my efforts to date - as you can see there are two sheets a front sheet which is what is displayed and a sheet that contains the examination details used to populate the various fields such as duration. There are three buttons at the end of the first exam Start, Stop and Reset. These all seem to work fine as long as you do not touch the sheet after starting the timer.

The code I currently have is as follows

Sub Call_Timer()
 Application.OnTime Now + TimeValue("00:00:01"), "UpdateTimer"
 End Sub

 Sub UpdateTimer()
 If ActiveCell.Address = "$B$16" Then
 Range("J15").Value = Range("J15").Value + 1 / 86400
 Call_Timer
 ElseIf ActiveCell.Address = "$B$17" Then
 Exit Sub
 ElseIf ActiveCell.Address = "$B$18" Then
 Range("J15").Value = 0
 End If
 End Sub

I also have the following on the sheet code

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
 UpdateTimer
 End Sub