'Return when the supplied field was changed.
Arguments:
kvs_key (`DjangoKeyValueStore.Key`): The field value to delete
Returns: datetime if there was a modified date, or None otherwise'

@contract(kvs_key=DjangoKeyValueStore.Key, returns='datetime|None')
def last_modified(self, kvs_key):

    field_object = self._cache.get(self._cache_key_for_kvs_key(kvs_key))
    if (field_object is None):
        return None
    else:
        return field_object.modified
