def METHOD_NAME(timestamp_ms):
    """
    Truncate a timestamp in milliseconds to have hour granularity.
    :param timestamp_ms: timestamp generated in the impression.
    :type timestamp_ms: int
    :returns: a timestamp with hour, min, seconds, and ms set to 0.
    :rtype: int
    """
    return timestamp_ms - (timestamp_ms % _TIME_INTERVAL_MS)