[FIX] GAMIFICATION: Fixed the Attribute Error

' 'str' object has no attribute 'strftime' '

addons/gamification/models/challenge.py

@@ -57,7 +57,7 @@ def start_end_date_for_period(period, default_start_date=False, default_end_date
         end_date = default_end_date
 
     if start_date and end_date:
-        return (start_date.strftime(DF), end_date.strftime(DF))
+        return (datetime.strftime(start_date, DF), datetime.strftime(end_date, DF))
     else:
         return (start_date, end_date)