'Return when the supplied field was changed.
Arguments:
kvs_key (`DjangoKeyValueStore.Key`): The key representing the cached field
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):

    try:
        return self._client.get(self.user.username, kvs_key.block_scope_id, fields=[kvs_key.field_name]).updated
    except self._client.DoesNotExist:
        return None
