def julian2num(j):
    """
    Convert a matplotlib date to a Julian days.

    Parameters
    ----------
    j : numpy.ndarray : int32
        Julian days.

    Returns
    -------
    num : numpy.ndarray : int32
        Number of days since 0001-01-01 00:00:00 UTC *plus* *one*.
    """
    return j - 1721424.5
