日期格式化和计算

日期格式化（这里是以LocalDate作为示例）

DateTimeFormatter df = DateTimeFormatter.ofPattern("yyyyMMdd");
LocalDate date = LocalDate.parse("20220101", df);
System.out.println(date);				// 2022-01-01
System.out.println(date.getYear());		// 2022
System.out.println(date.getMonth());	// JANUARY
System.out.println(date.getMonthValue());// 1
System.out.println(date.getDayOfYear()); // 1
System.out.println(date.getDayOfMonth());// 1
System.out.println(date.getDayOfWeek()); // SATURDAY

计算相差天数（年份、月份、分钟、秒、毫秒类似）

LocalDate d1 = LocalDate.parse("20220201", df);
LocalDate d2 = LocalDate.parse("20220301", df);
long daysDiff = ChronoUnit.DAYS.between(d1, d2);
System.out.println("日期差：" + daysDiff); // 日期差：28