from datetime import datetime


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)
    """


    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