code: @Override
      public String marshal(ZonedDateTime zonedDateTime) {
          if (null == zonedDateTime) {
              return null;
          }
  
          return zonedDateTime.withZoneSameInstant(ZoneOffset.UTC).format(formatter);
      }
docs: When marshalling, the provided zonedDateTime is normalized to UTC, so that the output is consistent between
  dates and time zones.
  
  @param zonedDateTime    the zonedDateTime to marshall
  @return                  the UTC-based date time in ISO format