def finalize_timeseries(ts, start):
    """
    Finalize timeseries.

    Drop all rows that have nan column values.  Set timeseries to begin
    at start.

    Parameters
    ----------
    ts : pd.DataFrame
        The timeseries of a symbol.
    start : datetime.datetime
        The start date for backtest.

    Returns
    -------
    datetime.datetime
        The start date.
    pd.DataFrame
        The timeseries of a symbol.
    """
    ts = ts.dropna()
    ts = ts[start:]
    start = ts.index[0]
    return ts, start