关于日期格式yyyy-MM-dd和YYYY-MM-dd

原文：https://blog.csdn.net/guotufu/article/details/85696296

开发的时候用到这个类进行日期的转换，想转换的日期格式为“YYYY/MM/dd”，结果在2018年12月30日那天转换出来的格式是2019/12/30，请看下面的代码：

public static void main(String[] args) {
DateTimeFormatter dtf = DateTimeFormatter.ofPattern("YYYY/MM/dd");
DateTimeFormatter dtf2 = DateTimeFormatter.ofPattern("yyyy/MM/dd");
LocalDate localDate = LocalDate.of(2018, 12, 30);
System.out.println("日期1："+dtf.format(localDate));
System.out.println("日期2："+dtf2.format(localDate));
}
以上代码出来的结果为：

所以同学们在用日期转换类的时候要注意了，下面列一下不同的字符表示的含义：

Pattern

All letters 'A' to 'Z' and 'a' to 'z' are reserved as pattern letters. The following pattern letters are defined: 

  Symbol  Meaning                     Presentation      Examples
  ------  -------                     ------------      -------
   G       era                         text              AD; Anno Domini; A
   u       year                        year              2004;
   y       year-of-era                 year              2004;
   D       day-of-year                 number            189
   M/L     month-of-year               number/text       7; 07; Jul; July; J
   d       day-of-month                number            10

   Q/q     quarter-of-year             number/text       3; 03; Q3; 3rd quarter
   Y       week-based-year             year              1996;
   w       week-of-week-based-year     number            27
   W       week-of-month               number            4
   E       day-of-week                 text              Tue; Tuesday; T
   e/c     localized day-of-week       number/text       2; 02; Tue; Tuesday; T
   F       week-of-month               number            3

   a       am-pm-of-day                text              PM
   h       clock-hour-of-am-pm (1-12)  number            12
   K       hour-of-am-pm (0-11)        number            0
   k       clock-hour-of-am-pm (1-24)  number            0

   H       hour-of-day (0-23)          number            0
   m       minute-of-hour              number            30
   s       second-of-minute            number            55
   S       fraction-of-second          fraction          978
   A       milli-of-day                number            1234
   n       nano-of-second              number            987654321
   N       nano-of-day                 number            1234000000

   V       time-zone ID                zone-id           America/Los_Angeles; Z; -08:30
   z       time-zone name              zone-name         Pacific Standard Time; PST
   O       localized zone-offset       offset-O          GMT+8; GMT+08:00; UTC-08:00;
   X       zone-offset 'Z' for zero    offset-X          Z; -08; -0830; -08:30; -083015; -08:30:15;
   x       zone-offset                 offset-x          +0000; -08; -0830; -08:30; -083015; -08:30:15;
   Z       zone-offset                 offset-Z          +0000; -0800; -08:00;

   p       pad next                    pad modifier      1

   '       escape for text             delimiter
   ''      single quote                literal           '
   [       optional section start
   ]       optional section end
   #       reserved for future use
   {       reserved for future use
   }       reserved for future use
 

原文：https://blog.csdn.net/tuesdayma/article/details/81669993

当天所在的周属于的年份，一周从周日开始，周六结束，只要本周跨年，那么这周就算入下一年。这个结论在正向转换的时候是没有问题的，但是在逆向转换的时候是有问题的。