日期时间救急锦囊： LocalDate / LocalDateTime / DateTimeFormatter

import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;

public class TimeQuickstart {
    public static void main(String[] args) {
        // 1. 打印当前日期 / 日期时间
        LocalDate today = LocalDate.now();
        LocalDateTime now = LocalDateTime.now();
        System.out.println(today); // 2026-08-03
        System.out.println(now);   // 2026-08-03T10:30:00.123

        // 2. 字符串 -> 时间对象（解析）
        LocalDate d = LocalDate.parse("2026-08-03"); // ISO 格式可直接解析
        LocalDateTime dt = LocalDateTime.parse("2026-08-03T10:30:00");

        // 3. 时间对象 -> 自定义格式（格式化）
        DateTimeFormatter fmt = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm");
        System.out.println(now.format(fmt)); // 2026-08-03 10:30

        // 4. 常用运算
        System.out.println(today.plusDays(7));   // 一周后
        System.out.println(dt.minusHours(2));    // 两小时前
        System.out.println(java.time.temporal.ChronoUnit.DAYS.between(
                LocalDate.of(2026, 1, 1), today)); // 相差天数
    }
}

LocalDate today = LocalDate.of(2026, 8, 3);

// 错误写法：计算结果没接住，today 纹丝不动
today.plusDays(7);
System.out.println(today); // 2026-08-03（不是一周后！）

// 正确写法：重新赋值
today = today.plusDays(7);
System.out.println(today); // 2026-08-10

import java.time.LocalDate;
import java.time.Period;
import java.time.MonthDay;
import java.time.temporal.ChronoUnit;

public class TimeComplete {
    public static void main(String[] args) {
        LocalDate launch = LocalDate.of(2025, 12, 31);
        LocalDate now = LocalDate.of(2026, 8, 3);

        // 比较：isBefore / isAfter / isEqual
        System.out.println(now.isAfter(launch)); // true

        // Period：按"年月日"人话表示两个日期的差
        Period age = Period.between(launch, now);
        System.out.println(age.getYears() + "年" + age.getMonths() + "个月");
        // 预期输出：0年7个月

        // ChronoUnit：按单一单位换算
        System.out.println(ChronoUnit.DAYS.between(launch, now));  // 215
        System.out.println(ChronoUnit.MONTHS.between(launch, now)); // 7

        // MonthDay：只关心月日，做生日/周年提醒
        MonthDay birthday = MonthDay.of(9, 1);
        LocalDate nextBirthday = birthday.atYear(now.getYear());
        if (nextBirthday.isBefore(now)) {
            nextBirthday = birthday.atYear(now.getYear() + 1);
        }
        System.out.println("距离下次生日还有 "
                + ChronoUnit.DAYS.between(now, nextBirthday) + " 天");
        // 预期输出：距离下次生日还有 29 天
    }
}

import java.time.LocalDate;
import java.time.MonthDay;
import java.time.temporal.TemporalAdjusters;

public class DateDrill {
    public static void main(String[] args) {
        // 练习一
        LocalDate today = LocalDate.now();
        System.out.println(today + " 是星期" + today.getDayOfWeek().getValue()
                + "，是一年中的第 " + today.getDayOfYear() + " 天");
        // 严格按中文习惯映射「一~日」可用 switch，此处保留 DayOfWeek 序数

        // 练习二：atYear 对平年把 2 月 29 日回落为 2 月 28 日（查证后再看答案）
        LocalDate leapDay = MonthDay.of(2, 29).atYear(2027);  // 2027 不是闰年
        System.out.println(leapDay);   // 预期输出：2027-02-28

        // 练习三
        System.out.println(today.minusYears(1));                            // 去年的今天
        System.out.println(today.with(TemporalAdjusters.lastDayOfMonth())); // 本月最后一天
    }
}