def timedelta(old, new):
    """Get timedelta between timepoints.

    Parameters
    ----------
    old : pd.DataFrame
        This is the older dataframe.
    new : pd.DataFrame
        This is the newer dataframe.

    Returns
    -------
    pd.Series
        The content will be timedelta values. Look into numpy for more options.

    """
    old = old.reset_index()
    old = old.set_index('Subject ID')
    new = new.reset_index()
    new = new.set_index('Subject ID')
    timedeltas = old['SCANDATE'] - new['SCANDATE']
    return timedeltas
