void contextLoads() {
        Date date = new Date();//獲取當(dāng)前時間
        System.out.println(date);
        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");//設(shè)置格式
        Calendar calendar = Calendar.getInstance(); //創(chuàng)建Calendar 的實(shí)例
        calendar.add(Calendar.DAY_OF_MONTH, -1); //當(dāng)前時間減去一天，即一天前的時間
        System.out.println(simpleDateFormat.format(calendar.getTime()));
        Calendar calendar2 = Calendar.getInstance();
        calendar2.add(Calendar.MONTH, -1);//當(dāng)前時間減去一個月，即一個月前的時間
        System.out.println(simpleDateFormat.format(calendar2.getTime()));
        Calendar calendar3 = Calendar.getInstance();
        calendar.add(Calendar.YEAR, -1);//當(dāng)前時間減去一年，即一年前的時間
        System.out.println(simpleDateFormat.format(calendar3.getTime()));
        System.out.println(calendar.getTimeInMillis());//返回當(dāng)前時間的毫秒數(shù)
    }

    /**
     * 獲取某天的時間,支持自定義時間格式
     *
     * @param simpleDateFormat 時間格式,yyyy-MM-dd HH:mm:ss
     * @param index            為正表示當(dāng)前時間加天數(shù)，為負(fù)表示當(dāng)前時間減天數(shù)
     * @return String
     */
    public static String getTimeDay(String simpleDateFormat, int index) {
        //設(shè)置時區(qū)
        TimeZone tz = TimeZone.getTimeZone("Asia/Shanghai");
        TimeZone.setDefault(tz);
        Calendar calendar = Calendar.getInstance();
        SimpleDateFormat fmt = new SimpleDateFormat(simpleDateFormat);
        calendar.add(Calendar.DAY_OF_MONTH, index);
        String date = fmt.format(calendar.getTime());
        return date;
    }
    /**
     * 第二種使用自定義方法
     */
    void contextLoads2() {
        System.out.println(getTimeDay("yyyy-MM-dd HH:mm:ss", -1));
    }

    void contextLoads3() {
        Date now = DateUtil.now();
        System.out.println("今天時間： " + DateUtil.formatDateTime(now));
        System.out.println("昨天時間： " + DateUtil.formatDateTime(DateUtil.addDate(now, -1)));
    }

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
/**
 * @author lichangyuan
 * @create 2021-12-15 11:30
 */
public class DateUtil {
    public DateUtil() {
    }
    public static Date now() {
        return new Date();
    }
    public static Date add(Date date, Integer field, Integer amount) {
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(date);
        calendar.add(field, amount);
        return calendar.getTime();
    }
    public static Date addDate(Date date, Integer days) {
        return add(date, 5, days);
    }
    public static String format(Date date, String format) {
        if (date == null) {
            return "";
        } else {
            if (isEmpty(format)) {
                format = "yyyy-MM-dd";
            }
            SimpleDateFormat sdf = new SimpleDateFormat(format);
            return sdf.format(date);
        }
    }
    public static String formatDateTime(Date date) {
        return format(date, "yyyy-MM-dd HH:mm:ss");
    }
    /**
     * 計(jì)算兩個日期相差天數(shù)
     *
     * @param smdate
     * @param bdate
     * @return
     * @throws ParseException
     */
    public static int daysBetween(Date smdate, Date bdate) {
        long betweenDays = 0;
        try {
            SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd  HH:mm:ss");
            smdate = sdf.parse(sdf.format(smdate));
            bdate = sdf.parse(sdf.format(bdate));
            Calendar cal = Calendar.getInstance();
            cal.setTime(smdate);
            long time1 = cal.getTimeInMillis();
            cal.setTime(bdate);
            long time2 = cal.getTimeInMillis();
            betweenDays = (time2 - time1) / (1000 * 3600 * 24);
        } catch (Exception ex) {
            System.out.println(ex.getMessage());
        }
        return Integer.parseInt(String.valueOf(betweenDays));
    }
}

    void test12() {
        //SimpleDateFormat中的parse方法可以把特定格式的String型的字符串轉(zhuǎn)換成date類型
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        try {
            Date newDate = sdf.parse("2022-12-01 13:00:00");
            System.out.println(newDate);
            SimpleDateFormat sdf2 = new SimpleDateFormat("yyyy:MM:dd HH-mm-ss");
            String format = sdf2.format(newDate);
            System.out.println(format);
        } catch (ParseException e) {
            e.printStackTrace();
        }
    }