There are a couple of ways to do this ...
Nice and simple is to select the control and send it the down key ...
but there is always the faint danger that the sendkeys may go elsewhere if you have a "clicky" user.
Safer way is to send the message directly to the control
[DllImport("user32.dll", SetLastError = true)]
private static extern int SendMessage(IntPtr hWnd, uint Msg, int wParam, int lParam);
const int WM_SYSKEYDOWN = 0x0104;
at the top of your class and then after your messagebox
SendMessage(dateTimePicker1.Handle, WM_SYSKEYDOWN, (int)Keys.Down, 0);