Option Explicit
'Определим объекты которые вазываяют подмену события
Public Frm As Form
Public MaskEdBoxCtr As MaskEdBox

'Определим необходимые константы
Const GWL_WNDPROC = - 4 &
Const WM_LBUTTONDBLCLK = &H203

'Объявления API-функций
Declare Function SetWindowLong Lib "user32" _
Alias "SetWindowLongA" (ByVal hwnd As Long, _
ByVal nIndex As Long, ByVal dwNewLong As Long) As Long
Declare Function CallWindowProc Lib "user32" _
Alias "CallWindowProcA" (ByVal lpPrevWndFunc As Long, _
ByVal hwnd As Long, ByVal Msg As Long, ByVal wParam As Long, _
ByVal lParam As Long) As Long

'Эта процедура заменит стандартную оконную
'процедуру на пользовательскую
Public Sub SetWndProc()
 Frm.hPrevWndProc = _
 SetWindowLong(MaskEdBoxCtr.hwnd, _
 GWL_WNDPROC, AddressOf WindowProc)
End Sub

'Пользовательская оконная процедура
Public Function WindowProc(ByVal hwnd As Long, _
 ByVal iMsg As Long, _
 ByVal wParam As Long, _
 ByVal lParam As Long) As Long
 Select Case iMsg
    Case WM_LBUTTONDBLCLK
    If MaskEdBoxCtr = "__.__.____" Then
    MaskEdBoxCtrl = Date
    Else
    MaskEdBoxCtr = "__.__.____"
    End If
 End Select

 'вызов оконной процедуры по умолчанию
 WindowProc = CallWindowProc( _
 Frm.hPrevWndProc, MaskEdBoxCtr.hwnd, _
 iMsg, wParam, lParam)
End Function

Option Explicit

Private m_hPrevWndProc As Long

'Свойство, содержащее адрес оконной процедуры по умолчанию
Public Property Get hPrevWndProc() As Long
 hPrevWndProc = m_hPrevWndProc
End Property

Public Property Let hPrevWndProc(Value As Long)
 m_hPrevWndProc = Value
End Property

Private Sub MaskEdBox1_GotFocus()
Set Frm = Me
Set MaskEdBoxCtr = Me.ActiveControl
SetWndProc
End Sub