code: public static String createGmtOffsetString(boolean includeGmt,
              boolean includeMinuteSeparator, int offsetMillis) {
          int offsetMinutes = offsetMillis / 60000;
          char sign = '+';
          if (offsetMinutes < 0) {
              sign = '-';
              offsetMinutes = -offsetMinutes;
          }
          StringBuilder builder = new StringBuilder(9);
          if (includeGmt) {
              builder.append("GMT");
          }
          builder.append(sign);
          appendNumber(builder, 2, offsetMinutes / 60);
          if (includeMinuteSeparator) {
              builder.append(':');
          }
          appendNumber(builder, 2, offsetMinutes % 60);
          return builder.toString();
      }
docs: Returns a string representation of an offset from UTC.
  
  <p>The format is "[GMT](+|-)HH[:]MM". The output is not localized.
  
  @param includeGmt true to include "GMT", false to exclude
  @param includeMinuteSeparator true to include the separator between hours and minutes, false
  to exclude.
  @param offsetMillis the offset from UTC
  
  @hide used internally by SimpleDateFormat