def epoch_to_timestamp(records):
    """
    Convert epoch timestamps to ISO 8601 (2015-01-04T09:30:21Z)

    :param records: List (of dictionaries)
    :return: List (of dictionaries)
    """
    from datetime import datetime
    for record in records:
        timestamp_keys = ['time_first', 'time_last']
        for key in timestamp_keys:
            if key in record:
                record[key] = datetime.fromtimestamp(record[key]).isoformat(
                    ) + 'Z'
    return records
