code: public static long convertToUtc(long millis, String tz)
      {
          long ret = millis;
          if(tz != null && tz.length() > 0)
          {
              DateTime dt = new DateTime(millis, DateTimeZone.forID(tz));
              ret = dt.withZoneRetainFields(DateTimeZone.forID("UTC")).getMillis();
          }
          return ret;
      }
docs: Returns the given date converted to UTC using the given timezone.
  @param millis The date to convert
  @param tz The timezone associated with the date
  @return The given date converted using the given timezone