Do not convert date if already a date

tailbone/grids/filters.py

@@ -675,6 +675,9 @@ class AlchemyDateFilter(AlchemyGridFilter):
         Convert user input to a proper ``datetime.date`` object.
         """
         if value:
+            if isinstance(value, datetime.date):
+                return value
+
             try:
                 dt = datetime.datetime.strptime(value, '%Y-%m-%d')
             except ValueError: