wxCalendarCtrl and wxDialog

I have a problem concerning following dialoge

(derived from wxDialog)
     def OnDaySelected(self, event):
     self.m_date = event.GetDate()
     print self.m_date
    (here it is ok; the currently selected date)

if wxOK == dlg.ShowModal():
    self.m_dateFrom = dlg.m_date
    print dlg.m_date
    (here, it is always 01.01.1970)
    (it doesn't keep the value)

What can be the reason?

Thank you in advance!

Francesco