How to combine a code and a macro

Try this,
I used a sheet names "Start". When you run the code, the code will delete all the sheets except sheets("Start")

Sub OPenMultipleWorkbooks()
'Open Multiple .csv files, this workbooks name is, 'Compare Workbook.xlsm'
    Dim wb As Workbook
    Dim GetFile As Variant

    Set wb = ThisWorkbook
    Application.DisplayAlerts = False
    
    For Each Sheet In Sheets
        If Sheet.Name <> "Start" Then Sheet.Delete
    Next Sheet
    ChDrive "C:"

    GetFile = Application.GetOpenFilename(FileFilter:="CSV (*.CSV), *.CSV", Title:="Open CSV File", MultiSelect:=True)
    Application.ScreenUpdating = False

    On Error Resume Next

    If GetFile <> False Then
        On Error GoTo 0
        For i = 1 To UBound(GetFile)

            Workbooks.Open Filename:=GetFile(i)
            Sheets(1).Move Before:=wb.Sheets(1)

        Next i
    End If
    CombineDts
End Sub
Sub CombineDts()
    Dim LstRw As Long, rng As Range, sh As Worksheet
    Dim Frmla As String

    Frmla = "=DATEVALUE(C2&"" / ""&D2&"" / ""&B2)"
    For Each sh In Sheets
        With sh
            If .Range("A1") = "ID" Then
                LstRw = .Cells(.Rows.Count, "A").End(xlUp).Row
                Set rng = .Range("A2:A" & LstRw)
                rng = Frmla
                rng.Value = rng.Value
                rng.NumberFormat = "dd/mm/yyyy"
                .Range("A1") = "Date"
                .Columns("B:D").Delete Shift:=xlToLeft
            End If
        End With
    Next sh
End Sub