def formatLongDuration(sec):
    """Formats a duration in days:hours format, preferable for very long durations.

    :type sec: int
    :param sec: duration in seconds
    :rtype: str
    :return: duration formatted in days:hours format."""
    def splitTime(seconds):
        minutes, seconds = divmod(seconds, 60)
        hour, minutes = divmod(minutes, 60)
        days, hour = divmod(hour, 24)
        return days, hour
    return "%02d:%02d" % splitTime(sec)