diff --git a/tryton/trytond/trytond/ir/lang.py b/tryton/trytond/trytond/ir/lang.py
index d6e61e3599..4f3954beef 100644
--- a/tryton/trytond/trytond/ir/lang.py
+++ b/tryton/trytond/trytond/ir/lang.py
@@ -542,6 +542,8 @@ class Lang(DeactivableMixin, ModelSQL, ModelView):
         if isinstance(value, datetime.date):
             for f, i, klass in (('%A', 6, Day), ('%B', 1, Month)):
                 for field, f in [('name', f), ('abbreviation', f.lower())]:
+                    if f not in format:
+                        continue
                     locale = klass.locale(self, field=field)
                     format = format.replace(f, locale[value.timetuple()[i]])
         if isinstance(value, datetime.time):