def format_interval(t):
    """
        Formats a number of seconds as a clock time, [H:]MM:SS
        
        Parameters
        ----------
        t  : int
        Number of seconds.
        Returns
        -------
        out  : str
        [H:]MM:SS
    """
    mins, s = divmod(int(t), 60)
    h, m = divmod(mins, 60)
    if h:
        return '{0:d}:{1:02d}:{2:02d}'.format(h, m, s)
    else:
        return '{0:02d}:{1:02d}'.format(m, s)