code: protected static final String formatUsingFormat(Long value, DateTimeFormat fmt) {
          if (value == null) {
              return "";
          }
          else {
              // midnight GMT
              Date date = new Date(0);
              // offset by timezone and value
              date.setTime(UTCDateBox.timezoneOffsetMillis(date) + value.longValue());
              // format it
              return fmt.format(date);
          }
      }
docs: Formats the value provided with the specified DateTimeFormat