def jd2mjd(jd):
    """
    Converts a Julian Date to a Modified Julian Date

    Parameters
    ----------
    jd : float (any numeric type)
        A julian date

    Returns
    -------
    mjd : float
        The Modified Julian Date (MJD) calculated from the input Julian Date

    Examples
    --------
    >>> jd2mjd(2455581.40429)
    55580.90429

    """
    return float(jd - 2400000.5)
