VBA countdown timer

At the end of the code you need to call another macro, that calls the countdown macro again, you also need a way to stop the code.

I just thought of placing the word "Stop" in D1 and the loop will stop.

Sub CountDwn()
    Calculate
    If [d1] = "Stop" Then Exit Sub
    Application.OnTime Now + TimeValue("00:00:01"), "CntDownTimer"

End Sub
Sub CntDownTimer()
    Dim startdate As Date, enddate As Date
    Dim days As Long, hours As Long, minutes As Long, seconds As Long
    startdate = Now()
    enddate = "31/12/2019 20:00:00"

    days = Int(DateValue(enddate) - DateValue(startdate))
    hours = Hour(TimeValue(enddate) - TimeValue(startdate))
    minutes = Minute(TimeValue(enddate) - TimeValue(startdate))
    seconds = Second(TimeValue(enddate) - TimeValue(startdate))

    Range("D8") = days
    Range("F8") = hours
    Range("H8") = minutes
    Range("J8") = seconds
    CountDwn
End Sub
Sub StopCntDwn()
    [d1] = "Stop"
End Sub

Try this way.

In Standard Module.

Sub CountDwn()
    Calculate
    Application.OnTime Now + TimeValue("00:00:01"), "CntDownTimer"
End Sub

Sub CntDownTimer()
    Dim startdate As Date, enddate As Date
    Dim days As Long, hours As Long, minutes As Long, seconds As Long
    startdate = Now()
    enddate = "31/12/2019 20:00:00"

    days = Int(DateValue(enddate) - DateValue(startdate))
    hours = Hour(TimeValue(enddate) - TimeValue(startdate))
    minutes = Minute(TimeValue(enddate) - TimeValue(startdate))
    seconds = Second(TimeValue(enddate) - TimeValue(startdate))

    Range("D8") = days
    Range("F8") = hours
    Range("H8") = minutes
    Range("J8") = seconds
    CountDwn
End Sub

Sub StopCntDwn()
    On Error Resume Next
    Application.OnTime Now + TimeValue("00:00:01"), "CntDownTimer", , False
End Sub

In ThisWorkbook.

Private Sub Workbook_Open()
    CountDwn
End Sub

Private Sub Workbook_BeforeClose(Cancel As Boolean)
    ThisWorkbook.Saved = True
    StopCntDwn
End Sub