Excel VBA: Timer Update

Hello Experts,

I need assistance with a timer feature I have on a userform I created in Excel.  I have cmd buttons:  START, STOP, and RESET on the form that point to certain cells in order to activate the timer.  START = A12,  STOP= B12, and RESET= C12 (sheet:  "Entry_Form" and the time in "h:mm:ss" format is displayed in D12 which populates my textbox= txtTime.

My issue is that the timer works intermittently and I don't know how to correct it or better yet figure out what's causing it to work sometimes and not others.  When I say that it doesn't work I'm meaning that the "txtTime" field doesn't populate and refresh real time once the timer is started.  I see it working fine on the worksheet in the desginated cells but not on the form.

There's 3 pieces of code that makes the timer work:

In the "Workbook":

Private Sub Workbook_SheetSelectionChange(ByVal Sh As Object, ByVal Target As Range)
UpdateTimer

End Sub

Select allOpen in new window

Module 1:

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

Select allOpen in new window

and

Sub UpdateTimer()
If ActiveCell.Address = "$A$12" Then
Range("D12").Value = Range("D12").Value + 1 / 86400
Call_Timer
ElseIf ActiveCell.Address = "$B$12" Then
Exit Sub
ElseIf ActiveCell.Address = "$C$12" Then
Range("D12").Value = 0
End If
End Sub

Select allOpen in new window

' code updates txtTime field on form to proper format

Private Sub txtTime_Change()

frmCFSTracker.txtTime = Format(Worksheets("entry_form").Range("d12"), "hh:mm:ss")

End Sub

Select allOpen in new window

Control source for "txtTime"= "D12"

NOTE:  When the workbook 1st opens, I enable Macros and select it as a trusted source but no matter what the timer may work or may not work.

Thank you for your help!
Sample-CFS-Tracker2.zip