anchor.diff:  <<painless-api-reference-shared-ChronoUnit, ChronoUnit>>
   to calculate the difference between two complex datetimes if supported.
   
  -===== Elapsed Time Examples
  +===== Datetime Difference Examples
   
   * Difference in milliseconds between two numeric datetimes
   +
  
positive.code: [source,Painless]
  ----
  String datetime = '1983-10-13T22:15:30Z';
  ZonedDateTime zdt = ZonedDateTime.parse(datetime);
  ----
  Note the parse method uses ISO 8601 by default.
  +
  * parse from RFC 1123
  +
  
negative.code:         ZonedDateTime.of(1983, 10, 13, 22, 15, 30, 0, ZoneId.of('Z'));
  DateTimeFormatter dtf = DateTimeFormatter.ofPattern(
          "'date:' yyyy/MM/dd 'time:' HH:mm:ss");
  String datetime = zdt.format(dtf); <1>
  ----
  <1> Note the use of a custom DateTimeFormatter.
  
  ==== Datetime Conversion
  
  