title: How to use date_add with two columns in pyspark?
body: <p>I have a dataframe with some columns:</p>
  
  <pre><code>+------------+--------+----------+----------+
  |country_name| ID_user|birth_date|      psdt|
  +------------+--------+----------+----------+
  |      Россия|16460783|       486|1970-01-01|
  |      Россия|16467391|      4669|1970-01-01|
  |      Россия|16467889|      6861|1970-01-01|
  |   Казахстан|16468013|      5360|1970-01-01|
  |      Россия|16471027|      6311|1970-01-01|
  |      Россия|16474162|      5567|1970-01-01|
  |      Россия|16476386|      4351|1970-01-01|
  |      Россия|16481067|      3831|1970-01-01|
  |   Казахстан|16485965|     -2369|1970-01-01|
  |    Германия|16486027|      5864|1970-01-01|
  +------------+--------+----------+----------+
  only showing top 10 rows
  </code></pre>
  
  <p>I need to add "psdt" with "birth_date". I wrote this code, but (sf.date_add) doesn't work:</p>
  
  <pre><code>resultbirthDF =(
          resultDF
          .select(sf.col("country_name"),
                  sf.col("ID_user"),
                  sf.col("birth_date"),
                  sf.lit(past_datetr).alias("psdt")
                 )
          .withColumn("birth_datetrue",sf.date_add(sf.to_date(sf.col("psdt")),sf.col("birth_date")))
      ).show(10)
  
  'Column' object is not callable
  Traceback (most recent call last):
    File "/volumes/disk1/yarn/local/usercache/livy/appcache/application_1573843665329_0786/container_e05_1573843665329_0786_01_000001/pyspark.zip/pyspark/sql/functions.py", line 1006, in date_add
      return Column(sc._jvm.functions.date_add(_to_java_column(start), days))
  </code></pre>
  
  <p>How to solve this problem?</p>