def json_iso_dttm_ser(obj, pessimistic: Optional[bool] = False):
    """
    json serializer that deals with dates

    >>> dttm = datetime(1970, 1, 1)
    >>> json.dumps({'dttm': dttm}, default=json_iso_dttm_ser)
    '{"dttm": "1970-01-01T00:00:00"}'
    """
    val = base_json_conv(obj)
    if val is not None:
        return val
    if isinstance(obj, (datetime, date, time, pd.Timestamp)):
        obj = obj.isoformat()
    else:
        if pessimistic:
            return 'Unserializable [{}]'.format(type(obj))
        else:
            raise TypeError(
                'Unserializable object {} of type {}'.format(obj, type(obj)))
    return obj
json serializer that deals with dates

    >>> dttm = datetime(1970, 1, 1)
    >>> json.dumps({'dttm': dttm}, default=json_iso_dttm_ser)
    '{"dttm": "1970-01-01T00:00:00"}'