code: private static String millisToOffset(int millis) {
          StringBuilder sb = new StringBuilder(7);
          if (millis >= 0) {
              sb.append('+');
          } else {
              sb.append('-');
              millis = -millis;
          }
          int hour, min, sec;
          int t = millis / 1000;
  
          sec = t % 60;
          t = (t - sec) / 60;
          min = t % 60;
          hour = t / 60;
  
          sb.append(numToString(hour, 2));
          sb.append(numToString(min, 2));
          sb.append(numToString(sec, 2));
  
          return sb.toString();
      }
docs: /*
  Convert milliseconds to RFC2445 utc-offset string