anchor.diff:          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);
  -----
  -Note the use of a custom DateTimeFormatter.
  +String datetime = zdt.format(dtf); <1>
  +----
  +<1> Note the use of a custom DateTimeFormatter.
   
   ==== Datetime Conversion
   
  
positive.code: ----
  ZonedDateTime zdt =
          ZonedDateTime.of(1983, 10, 13, 22, 15, 30, 0, ZoneId.of('Z'));
  String datetime = zdt.format(DateTimeFormatter.ISO_INSTANT);
  ----
  Note the use of a built-in DateTimeFormatter.
  +
  * format to a custom format
  +
  
negative.code: <<painless-api-reference-shared-ChronoUnit, ChronoUnit>>
  to calculate the difference between two complex datetimes if supported.
  
  ===== Datetime Difference Examples
  
  * Difference in milliseconds between two numeric datetimes
  +
  