def to_iso_date(timestamp):
    """Convert a UTC timestamp to an ISO8601 string.

    datetime instances can be constructed in alternate timezones. This
    function assumes that the given timestamp is in the UTC timezone.

    Args:

        timestamp(datetime): A datetime object in the UTC timezone.

    Returns:

        An ISO8601 formatted string representing the timestamp.

    """
    return timestamp.isoformat() + 'Z'