title: No matching signature for function TIMESTAMP_DIFF.... error on BigQuery
body: <p>I'm trying to get the timestamp difference between two date fields in my table. I know both dates are on TIMESTAMP format, but when I tried using TIMESTAMP_DIFF function I get an error saying "No matching signature for function TIMESTAMP_DIFF for argument types: STRING, STRING, DATE_TIME_PART. Supported signature: TIMESTAMP_DIFF(TIMESTAMP, TIMESTAMP, DATE_TIME_PART) at [27:8]"</p>
  
  <p>I also tried formatting them again in the query(as done on the example for FIRST_VALUE(): <a href="https://cloud.google.com/bigquery/docs/reference/standard-sql/functions-and-operators#datetime_diff" rel="nofollow noreferrer">https://cloud.google.com/bigquery/docs/reference/standard-sql/functions-and-operators#datetime_diff</a> , and then it showed the same error but for FORMAT_TIMESTAMP.</p>
  
  <p><a href="https://i.stack.imgur.com/vdKLJ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/vdKLJ.png" alt="enter image description here"></a></p>
  
  <p>Any ideas what I could do to fix this or to get the time difference between two fields for each row?</p>