Countdown Timer Pause and Resume Button

Maybe like this.

Add three buttons from the Control Toolbox, and name them btnStart, btnPause, and btnResume. Then put this code in the Sheet module:

Option Explicit

Dim datNext         As Date

Sub btnStart_Click()
    Range("Time").Value = #12:05:00 AM# ' initial value
    If datNext <> 0 Then Application.OnTime datNext, Me.Name & ".Tick", , False
    datNext = Now + #12:00:01 AM#
    Application.OnTime datNext, Me.Name & ".Tick"
End Sub

Sub btnPause_Click()
    If datNext <> 0 Then
        Application.OnTime datNext, Me.Name & ".Tick", , False
        datNext = 0
    End If
End Sub

Sub btnResume_Click()
    datNext = Now + #12:00:01 AM#
    Application.OnTime datNext, Me.Name & ".Tick"
End Sub

Sub Tick()
    With Range("Time")
        Select Case .Value
            Case 0
                ' do nothing
            Case Is > #12:00:01 AM#
                .Value = .Value - #12:00:01 AM#
                datNext = Now + #12:00:01 AM#
                Application.OnTime datNext, Me.Name & ".Tick"
            Case Else
                .Value = 0
        End Select
    End With
End Sub