code: def date_add(start, days):
      
      sc = SparkContext._active_spark_context
      return Column(sc._jvm.functions.date_add(_to_java_column(start), days))
docs: 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))]