def sdssjd2mjd(sdssjd):
    """
    Converts a SDSS Julian Date to a Modified Julian Date

    Parameters
    ----------
    sdssjd : float (any numeric type)
        The SDSS Julian Date

    Returns
    -------
    mjd : float
        The Modified Julian Date calculated from the input SDSS Julian Date

    Notes
    -----
    - The SDSS Julian Date is a convenience used at APO to prevent MJD from rolling
        over during a night's observation.

    Examples
    --------
    >>> sdssjd2mjd(55581.20429)
    55580.90429

    """
    return sdssjd - 0.3