A technique for storing a JSON-serializable datetime as a computed property so that mapreduce can filter against it.

datetime_mapreduce.py

class UserData(db.Model):
    birthdate = db.DateProperty(indexed=False)

    def compute_birthdate_str(self):
        if self.birthdate:
            return self.birthdate.isoformat()
        return self.birthdate

    # You will probably need to add this to index.yaml
    # - kind: UserData
    #   properties:
    #   - name: birthdate_str
    #   - name: __scatter__
    birthdate_str = db.ComputedProperty(compute_birthdate_str, indexed=True)

    @staticmethod
    def run_mapreduce():
        mapreduce_id = mapreduce.control.start_map(
            name="SendParentEmails",
            handler_spec="emails.handlers.send_parent_email",
            reader_spec=(
                "third_party.mapreduce.input_readers.DatastoreInputReader"),
            mapper_parameters={
                "input_reader": {
                    "entity_kind": "UserData",
                    # NOTE: You can only use "=" as the second argument
                    "filters": [("birthdate_str", "=", "2013-05-16")]
                },
            },
            mapreduce_parameters={},
            shard_count=32)
        return "MapReduce job launched: %s" % mapreduce_id