android gmt 시간 구하기

public static String convertCurrentTime(String regDate) {

        if (StringUtils.isEmpty(regDate))
            return null;

        String result = regDate;
        Logger.log("#12112 regDate->"+regDate);
        try {

            // 2017.02.03 경률 수정
            String dif_time = "";
            String sign_s = "";
            int different_time_h = 0;   // 시차 (시)
            int different_time_m = 0;   // 시차 (분)

            // 시간차 포맷
            SimpleDateFormat hour_dif_format = new SimpleDateFormat("HH:mm");
            Date hour_dif = hour_dif_format.parse("00:00");
            Calendar cal_hour_dif = Calendar.getInstance();
            cal_hour_dif.setTime(hour_dif);

            // GMT 구함
            Date date = new Date();
            DateFormat df = new SimpleDateFormat("z");
            // +,- 구함
            char sign = df.format(date).charAt(3);
            // gmt 구함 GMT+1400
            String gmt_hour = df.format(date).substring(4, 6);
            String gmt_min = df.format(date).substring(7, 9);
            int hour = Integer.parseInt(gmt_hour);
            int min = Integer.parseInt(gmt_min);

            //gmt포맷변경
            SimpleDateFormat gmtFormat = new SimpleDateFormat("HH:mm");
            Date gmtDate = gmtFormat.parse(gmt_hour + ":" + gmt_min);
            Calendar cal_gmt = Calendar.getInstance();
            cal_gmt.setTime(gmtDate);
            Logger.log("#12112 cal_gmt->"+cal_gmt.getTime());

            if (sign == '+') {
                if (hour >= 9) { //+차이
                    cal_hour_dif.add(Calendar.HOUR, hour);
                    cal_hour_dif.add(Calendar.MINUTE, min);
                    cal_hour_dif.add(Calendar.HOUR, -9);
                    sign_s = "+:";
                } else {      //-차이
                    cal_hour_dif.add(Calendar.HOUR, 9);
                    cal_hour_dif.add(Calendar.HOUR, -hour);
                    cal_hour_dif.add(Calendar.MINUTE, -min);
                    sign_s = "-:";
                }
                dif_time = sign_s + hour_dif_format.format(cal_hour_dif.getTime());;
                different_time_h = Integer.parseInt(dif_time.split(":")[1]);
                different_time_m = Integer.parseInt(dif_time.split(":")[2]);
            } else {
                cal_hour_dif.add(Calendar.HOUR, 9);
                cal_hour_dif.add(Calendar.HOUR, hour);
                cal_hour_dif.add(Calendar.MINUTE, min);
                sign_s = "-:";

                dif_time = sign_s + hour_dif_format.format(cal_hour_dif.getTime());
                different_time_h = Integer.parseInt(dif_time.split(":")[1]);
                different_time_m = Integer.parseInt(dif_time.split(":")[2]);
            }


//                 시간 계산할 수 있게 포맷변경
            SimpleDateFormat sdformat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
            DateFormat sdFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
            Date tempDate = sdFormat.parse(regDate);
            Calendar cal = Calendar.getInstance();
            cal.setTime(tempDate);
//
//                //시간계산
            if (dif_time.split(":")[0].equals("-")) {
                different_time_h *= -1;
                different_time_m *= -1;
            }

            cal.add(Calendar.HOUR, different_time_h);
            cal.add(Calendar.MINUTE, different_time_m);
            result = sdformat.format(cal.getTime());

        } catch (Exception e) {
            Logger.error("date error -> ", e);
            e.printStackTrace();
        }

        return result;
    }