title: Pyspark: Convert Column from String Numbers to Timestamp Type
body: <p>I am looking to convert Numbers in string format to timestamp.</p>
  <p>Sample:</p>
  <pre><code>+--------+-------------------+
  |date_str|expected date      |
  +--------+-------------------+
  |1.63E+15|1991-11-25 13:39:00|
  |1.63E+15|1991-11-25 13:40:00|
  |1.63E+15|1991-11-25 13:41:00|
  +--------+-------------------+
  </code></pre>
  <p>I have tried using <code>to_timestamp()</code> but returning Null values. also converted to <code>unix_timestamp</code> but no luck.</p>
  <pre><code>sdf1.select(F.to_timestamp(sdf1.date_str, 'yyyy/MM/dd HH:mm:ss').alias('date')).show()
  sdf1.select(to_timestamp('date_str', 'dd/MM/yyyy HH:mm:ss')).show()
  </code></pre>