Returns the date that is `days` days after `start`

    >>> df = spark.createDataFrame([('2015-04-08',)], ['dt'])
    >>> df.select(date_add(df.dt, 1).alias('next_date')).collect()
    [Row(next_date=datetime.date(2015, 4, 9))]

def date_add(start, days):
    """
    Returns the date that is `days` days after `start`

    >>> df = spark.createDataFrame([('2015-04-08',)], ['dt'])
    >>> df.select(date_add(df.dt, 1).alias('next_date')).collect()
    [Row(next_date=datetime.date(2015, 4, 9))]
    """
    sc = SparkContext._active_spark_context
    return Column(sc._jvm.functions.date_add(_to_java_column(start), days))