diff --git a/themis/importexport/utils.py b/themis/importexport/utils.py
index db9e7ba..6091b7d 100644
--- a/themis/importexport/utils.py
+++ b/themis/importexport/utils.py
@@ -113,7 +113,10 @@ class UtilityView(BrowserView):
 
 def convert(value, attr=None, context=None):
     if isinstance(value, datetime.date):
-        value = value.strftime('%Y-%m-%d')
+        try:
+            value = value.strftime('%Y-%m-%d')
+        except ValueError:
+            value = None
     elif isinstance(value, datetime.datetime):
         value = value.strftime('%Y-%m-%d %H:%M:%S')
     elif isinstance(value, plone.namedfile.file.NamedBlobFile) or \