def build_target(df_, number_of_periods_ahead, target):
    """ 
    takes a series, turned it into a dataframe, and adds a new column called target
    This column is the input series, lagged number_of_periods_ahead into the future
    """
    
    # the target column will be the input series, lagged into the future
    df_ = df_.copy()
    df_['target'] = df_[target].shift(-number_of_periods_ahead)
    return df_