Private Sub UserForm_Activate()
    Dim wksData As Worksheet
    Dim fdom() As Date
    Dim Lastrow As Long
    
    'set object variable
    Set wksData = Worksheets("Sheet1")
    Lastrow = wksData.Cells(Rows.Count, 1).End(xlUp).Row
    'reset array sizes to hold the data
    ReDim fdom(Lastrow) As Date
    ReDim ldom(Lastrow) As Date
    
    'fill the array with intial dates of the months in the first cell
    
    For i = 1 To Lastrow
        fdom(i) = DateSerial(Year(wksData.Cells(i, 1).Value), Month(wksData.Cells(i, 1).Value), 1)
    Next i
    
    'put array on worksheet
    For i = 1 To Lastrow
        wksData.Cells(i, 3).Value = Format(fdom(i), "mmm YYYY")
    Next i
    
    'put the dates onto the worksheet then filter them
    With Range(wksData.Cells(1, 3), wksData.Cells(Lastrow, 3))
        .Sort Cells(1, 3), xlAscending
        .AdvancedFilter xlFilterCopy, , Cells(1, 6), True
    End With
    
    'get the new number of rows to fill array with
    Lastrow = wksData.Cells(Rows.Count, 6).End(xlUp).Row
    'resize array to new size this also clears the array
    ReDim fdom(Lastrow) As Date
    
    For i = 1 To Lastrow
        'Fill when list is filtered if statement to remove inital value if applicable
        If Not (i = 1 And wksData.Cells(1, 6).Value = wksData.Cells(2, 6).Value) Then
            ListBox1.AddItem Format(wksData.Cells(i, 6).Value, "mmm yyyy")
        End If
    Next i
    
    'clear working of of worksheet
    wksData.Cells(1, 3).EntireColumn.Clear
    wksData.Cells(1, 6).EntireColumn.Clear
    

End Sub