Hallo,

der relevante Teil in Modul1 könnte dafür so aussehen:

Option Explicit

Private Const WORKSHEET_NAME As String = "Tabelle1"
Private Const WORKSHEET_COLUMN As String = "A"
Private Const WORKSHEET_COLUMN_STATUS As String = "B"
Private Const WORKSHEET_COLUMN_STATUS_DETAILED As String = "C"
Private Const WORKSHEET_COLUMN_STATUS_TIME As String = "D"
Private Const TIME_FORMAT As String = "dd.mm.yyyy Hh:Nn:Ss" 'https://msdn.microsoft.com/de-de/library/office/gg251755.aspx
    
Public Sub add_datetime(ByVal Statustext As String, Optional ByVal Detailtext As String = vbNullString, Optional ByVal markieren As Boolean = False)
    Dim wks As Worksheet
    Dim l As Long
    Set wks = Worksheets(WORKSHEET_NAME)
        
    With wks
        l = .Cells(.Rows.Count, WORKSHEET_COLUMN).End(xlUp).Row + 1
        .Cells(l, WORKSHEET_COLUMN).Value = Format(Now, TIME_FORMAT)
        .Cells(l, WORKSHEET_COLUMN_STATUS).Value = Statustext
        .Cells(l, WORKSHEET_COLUMN_STATUS_DETAILED).Value = Detailtext
        If Not .Cells(l - 1, WORKSHEET_COLUMN) = vbNullString Then
            .Cells(l, WORKSHEET_COLUMN_STATUS_TIME).Value = DateDiff("s", TimeValue(.Cells(l - 1, WORKSHEET_COLUMN).Value), TimeValue(.Cells(l, WORKSHEET_COLUMN).Value), vbUseSystemDayOfWeek, vbUseSystem)
        End If
        .UsedRange.Columns.AutoFit
        If markieren Then
            .Range(.Cells(l, WORKSHEET_COLUMN), .Cells(l, WORKSHEET_COLUMN_STATUS_DETAILED)).Interior.Color = vbRed
        End If
    End With
        
    Set wks = Nothing
End Sub

Sub Schaltfläche2_Klicken()
    Call add_datetime("i.O.")
End Sub

Sub Schaltfläche3_Klicken()
    UserForm2.Show vbModeless
End Sub

Die Differenz zwischen den Einträgen wird in Sekunden ausegegeben.

Gruß