How to send a date to SysMonthCal32 of running external application?

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VB

.NET

.NET4

API

I'm trying to figure out how to send/set a date to SysMonthCal32 of a external application. I have the correct windowhandle and can set present textboxes, checkboxes and buttons, but somehow I can't get it to work for the MonthCalendar.

My code:

Imports System.Runtime.InteropServices
Imports System.Text

Public Class Form1
   
    'MonthCalendar
    Public Const MCM_SETCURSEL = &H1002

    Dim hWnd As IntPtr
    Dim sClassName As StringBuilder
    Dim length As Integer
    Dim sb As New System.Text.StringBuilder

Public Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        Dim ChildHandle As IntPtr
        Dim ChildHandle2 As IntPtr
        Dim ChildHandle3 As IntPtr
        Dim ButtonHandle As IntPtr

        Dim MainWindowHandle As IntPtr
        MainWindowHandle = FindWindow(Nothing, TextBox3.Text)
        hWnd = Nothing
        hWnd = FindWindowEx(MainWindowHandle, 0&, vbNullString, vbNullString)
        hWnd = FindWindowEx(MainWindowHandle, hWnd, vbNullString, vbNullString)
        hWnd = FindWindowEx(MainWindowHandle, hWnd, vbNullString, vbNullString)

        ChildHandle = FindWindowEx(hWnd, 0&, vbNullString, vbNullString)
        ChildHandle = FindWindowEx(hWnd, ChildHandle, vbNullString, vbNullString)

        ChildHandle2 = FindWindowEx(ChildHandle, IntPtr.Zero, "WindowsForms10.Window.8.app.0.33c0d9d", "Date frame to search")

        ChildHandle3 = FindWindowEx(ChildHandle2, 0&, vbNullString, vbNullString)
        ButtonHandle = FindWindowEx(ChildHandle2, ChildHandle3, vbNullString, vbNullString)

        '### TO CHECK FOR CORRECT WINDOW/HANDLE ###
        sClassName = New StringBuilder("", 256)
        Call GetClassName(ButtonHandle, sClassName, 256)
        TextBox1.Text = sClassName.ToString
                
        length = GetWindowTextLength(ButtonHandle)
        sb = New System.Text.StringBuilder("", length)
        GetWindowText(ButtonHandle, sb, sb.Capacity + 1)
        TextBox2.Text = sb.ToString
        '###

        Dim seldate As Date = Format(CDate("24-08-2013"), "dd-MM-yyyy")
        Dim setdate As SYSTEMTIME
        With setdate
            .wYear = Year(seldate)
            .wMonth = Month(seldate)
            .wDayOfWeek = Weekday(seldate, vbSunday) - 1
            .wDay = seldate.Day
            .wHour = Hour(seldate)
            .wMinute = Minute(seldate)
            .wSecond = Second(seldate)
            .wMilliseconds = 0
        End With

        If ButtonHandle <> 0 Then
            SendMessageDate(ButtonHandle, MCM_SETCURSEL, 0&, setdate)
        End If

End Sub

    'SENDMESSAGEDATE
   Private Declare Function SendMessageDate Lib "user32.dll" Alias "SendMessageA" (ByVal hWnd As Long, ByVal wMsg As Long, ByVal wParam As Long, ByRef lParam As SYSTEMTIME) As Long
    
   Public Structure SYSTEMTIME
        Dim wYear As Integer
        Dim wMonth As Integer
        Dim wDayOfWeek As Integer
        Dim wDay As Integer
        Dim wHour As Integer
        Dim wMinute As Integer
        Dim wSecond As Integer
        Dim wMilliseconds As Integer
   End Structure

End Class

The error I'm getting is on the SendMessageDate:
PInvokeStackImbalance was detected!

Does anyone have any ideas? Thanks in advance!