import typing


def _generate_datetime_placeholder(schema: dict, path: typing.List[str]) -> typing.Union[dict, None]:
    """
    Generates a placeholder datetime object based on an object schema.

    :param schema: the sampledb object schema
    :param path: the path to this subschema
    :return: the generated object or None, if there is no default datetime
    """
    if 'default' not in schema:
        return None
    utc_datetime = schema['default']
    return {
        '_type': 'datetime',
        'utc_datetime': utc_datetime
    }