title: Update Processor returns empty String: Converting string date to long in NIFI
body: <p>Converting a string date with the format mentioned on the image to a number (long) but the output I get is empty string. </p>
  
  <p>Using a JSON reader and writer; </p>
  
  <p>where in input JSON it is a string and in the output JSON it is of type long.</p>
  
  <p>Tried to keep the output JSON type as a String and tried to evaluate the following expression but that was also empty string</p>
  
  <p>${DATE1.value:toDate('yyyy-MM-dd HH:mm:ss'):toNumber():toString()}</p>
  
  <p>Sample data trying to convert: {"DATE1" : "2018-01-17 00:00:00"}</p>
  
  <p>Tried to follow the solution on this <a href="https://community.hortonworks.com/questions/109714/nifi-processor-convert-stringdatetime-format-to-lo.html" rel="nofollow noreferrer">link</a> but still getting empty string.</p>
  
  <p><a href="https://i.stack.imgur.com/2d1va.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/2d1va.png" alt="enter image description here"></a></p>