Let wxVariantDataDateTime::Write() and Read() work with invalid dates (otherwise wxVariant::GetString() will assert when value is an invalid wxDateTime)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@58054 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/variant.cpp

@@ -1223,7 +1223,11 @@ bool wxVariantDataDateTime::Write(wxSTD ostream& str) const
 
 bool wxVariantDataDateTime::Write(wxString& str) const
 {
+    if ( m_value.IsValid() )
         str = m_value.Format();
+    else
+        str = wxT("Invalid");
+
     return true;
 }
 
@@ -1239,6 +1243,12 @@ bool wxVariantDataDateTime::Read(wxSTD istream& WXUNUSED(str))
 
 bool wxVariantDataDateTime::Read(wxString& str)
 {
+    if ( str == wxT("Invalid") )
+    {
+        m_value = wxInvalidDateTime;
+        return true;
+    }
+
     if(! m_value.ParseDateTime(str))
         return false;
     return true;