def convert_datetimes_to_seconds(start_date, datetimes):
    """ Converts difference in datetimes to total elapsed seconds.

    Parameters
    ----------
    start_date : datetime object
        Start date to use for calculating total elapsed seconds.
    datetimes : list of datetimes
        List of datetimes to calculate total elapsed seconds for.

    Returns
    -------
    list of float
        Total elapsed seconds from start_date for each element in datetimes list.
    """
    datetimes_seconds = [(dt - start_date).total_seconds() for dt in datetimes]

    return datetimes_seconds