def sec_to_str(sec):
    """
        Return a nice string given a number of seconds, such as 21:53. Takes
        into account hours, minutes, and seconds.
    """
    s = ""
    
    if sec < 0:
        return "Unknown"
    
    hours = sec / 3600
    min = (sec % 3600) / 60
    sec = sec % 60
    
    if hours:
        s += str(hours) + ":"
    
    if min and min < 10 and hours:
        s += "0" + str(min) + ":"
    elif min:
        s += str(min) + ":"
    
    if sec < 10 and min:
        s += "0"
    s += str(sec)
    
    return s