try to avoid ValueError because of erroneous date

composer/schema/field.py

@@ -1,5 +1,5 @@
 #
-#  Copyright (c) 2013 Helmut Merz helmutm@cy55.de
+#  Copyright (c) 2016 Helmut Merz helmutm@cy55.de
 #
 #  This program is free software; you can redistribute it and/or modify
 #  it under the terms of the GNU General Public License as published by
@@ -315,7 +315,10 @@ class DateFieldInstance(NumberFieldInstance):
         if langInfo:
             locale = locales.getLocale(langInfo.language)
             fmt = locale.dates.getFormatter(*format)
-            return fmt.format(value)
+            try:
+                return fmt.format(value)
+            except ValueError:
+                return str(value)
         return str(value)
 
     def unmarshall(self, value):