1. 현재 날짜

    public void getCurrentDate() {
        if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.O) {
            LocalDate now = LocalDate.now();
            this.nowYear = now.getYear();
            this.nowMonth = now.getMonth().getValue();
            Log.d("@@@@", "nowMonth : "+ nowMonth);
            this.nowDay = now.getDayOfMonth();
        } else {
            Date now = new Date();
            SimpleDateFormat formatterYear = new SimpleDateFormat("yyyy");
            SimpleDateFormat formatterMonth = new SimpleDateFormat("MM");
            SimpleDateFormat formatterDay = new SimpleDateFormat("dd");
            int formatedNowYear = Integer.parseInt(formatterYear.format(now));
            int formatedNowMonth = Integer.parseInt(formatterMonth.format(now));
            int formatedNowDay = Integer.parseInt(formatterDay.format(now));
            Log.d("@@@@", formatedNowYear+" "+formatedNowMonth+" "+formatedNowDay);
            this.nowYear = formatedNowYear;
            this.nowMonth = formatedNowMonth;
            this.nowDay = formatedNowDay;
        }
    }

2. 현재시간

public void getCurrentTime() {
        if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.O) {
            LocalTime now = LocalTime.now();
            this.nowHour = now.getHour();
            this.nowMinute = now.getMinute();
        } else {
            Date now = new Date();
            SimpleDateFormat formatterHour = new SimpleDateFormat("HH");
            SimpleDateFormat formatterMinute = new SimpleDateFormat("mm");
            int formatedNowHour = Integer.parseInt(formatterHour.format(now));
            int formatedNowMinute = Integer.parseInt(formatterMinute.format(now));
            Log.d("@@@@", formatedNowHour + " " + formatedNowMinute);
            this.nowHour = formatedNowHour;
            this.nowMinute = formatedNowMinute;
        }
    }

3. AM, PM 시간에 따른

1) AM, PM

    //메시지 예약 시간 Am Pm
    public String getAmPmTen(int hour) {
        String amPm;
        if(hour >= 12){
            amPm = getString(R.string.pm_picker);
        } else{
            amPm = getString(R.string.am_picker);
        }
        return amPm;
    }

2) 24시간 -> 12시간

    //메시지 예약 시간 24시간 -> 12시간 formatter
    public int getFormattedHour (int hour) {
        if(hour >= 13){
            hour = hour - 12;
        }
        return hour;
    }

4. 10분뒤 날짜 시간

    //10분뒤 날짜 시간
    public void tenMinuteLater(){
        Calendar cal = Calendar.getInstance();
        SimpleDateFormat formatterYear = new SimpleDateFormat("yyyy");
        SimpleDateFormat formatterMonth = new SimpleDateFormat("MM");
        SimpleDateFormat formatterDay = new SimpleDateFormat("dd");
        SimpleDateFormat formatterHour = new SimpleDateFormat("HH");
        SimpleDateFormat formatterMinute = new SimpleDateFormat("mm");
        cal.add(Calendar.MINUTE, 10);

        int formatedNowYear = Integer.parseInt(formatterYear.format(cal.getTime()));
        int formatedNowMonth = Integer.parseInt(formatterMonth.format(cal.getTime()));
        int formatedNowDay = Integer.parseInt(formatterDay.format(cal.getTime()));
        int formatedNowHour = Integer.parseInt(formatterHour.format(cal.getTime()));
        int formatedNowMinute = Integer.parseInt(formatterMinute.format(cal.getTime()));

        this.nowYear = formatedNowYear;
        this.nowMonth = formatedNowMonth;
        this.nowDay = formatedNowDay;
        this.nowHour = formatedNowHour;
        this.nowMinute = formatedNowMinute;
    }

5. 시간 날짜 DATE로 포맷

1) 날짜 DATE로 포맷

    //finalTime formatter
    public String getFinalTime(int mH, int mM){
        finalTime = String.format("%02d", mH)+"시 "+String.format("%02d", mM)+"분 ";
        Log.d(TAG + "@@@@ getFinalTime", "finalTime: " + finalTime);
        return finalTime;
    }

2) 시간 DATE로 포맷

    //finalDate formatter
    public String getFinalDate(int mY, int mM, int mD){
        finalDate = String.format("%04d", mY)+"년 "+String.format("%02d", mM)+"월 "+String.format("%02d", mD)+"일 ";
        Log.d(TAG + "@@@@ getFinalDate ", "finalDate: " + finalDate);
        return finalDate;
    }

3) 시간+날짜 DATE로 포맷

finalDateTime = finalDate + "" + finalTime;

finalDateTime을 mFinalDateTime 에 넣어준다
finalDate은 getFinalDate
finalTime은 getFinalTime

    public Date stringToDate(String mFinalDateTime){
        Date dateFinalDateTime = null;
        SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy년 MM월 dd일 HH시 mm분");

        try {
            dateFinalDateTime = dateFormat.parse(mFinalDateTime);
            Log.d(TAG + "@@@@", "dateFinalDateTime: " + dateFinalDateTime );
            Log.d(TAG + "@@@@", "mFinalDateTime: " + mFinalDateTime );
        } catch (Exception e){ e.printStackTrace(); }

        return dateFinalDateTime;
    }

참고
https://sseong66.tistory.com/58
https://hianna.tistory.com/609