objExcel = System.Runtime.InteropServices.Marshal.GetActiveObject("Excel.Application")

Set wsh = VBA.CreateObject("WScript.Shell")
arg = ActiveWorkbook.Path & ";" & ActiveWorkbook.Name & ";" & ActiveSheet.Name & ";" & Target.Address
' Wait for the shelled application to finish:
errorCode = wsh.Run(strPathDatePicker & " " & arg, windowStyle, waitOnReturn)

Public Sub frmDatePicker_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    Dim Par() As String
    Dim strTemp As String

    Try
        Dim arg = Environment.GetCommandLineArgs
        strTemp = arg(1)
        Par = Split(strTemp, ";")

        'Split arg, to get the Information from the Excel Workbook
        strWbPath = Par(0)
        strWbName = Par(1)
        strWsName = Par(2)
        strAdresse = Par(3)

    Catch ex As Exception
        Me.Close()
    End Try
End Sub

Private Sub MonthCalendar1_DateSelected(sender As Object, e As DateRangeEventArgs) Handles MonthCalendar1.DateSelected
    Dim objExcel As Excel.Application
    Dim wb As Excel.Workbook
    Dim ws As Excel.Worksheet

    Try
        'Get the Excel Object
        objExcel = System.Runtime.InteropServices.Marshal.GetActiveObject("Excel.Application")


        For Each wb In objExcel.Workbooks
            If wb.Name = strWbName Then
                ws = wb.Sheets(strWsName)
                ws.Range(strAdresse.ToString).Value = e.Start()
                Me.Close()
            End If

        Next
    Catch ex As Exception
        MessageBox.Show(ex.Message)
        Me.Close()
    End Try

End Sub

Dim o As Object = GetObject("Book 1.xls")
Dim wb As Excel.Workbook = TryCast(o, Excel.Workbook)

arg = ActiveWorkbook.Path & ";" & ActiveWorkbook.Name & ";" & ActiveSheet.Name & ";" & Target.Address

arg = ThisWorkbook.Path & ";" & ThisWorkbook.Name & ";" & ActiveSheet.Name & ";" & Target.Address