YYYYMMdd 和 yyyyMMdd 的區別

來自 https://www.v2ex.com/t/633650

YYYY 是表示：當天所在的周屬於的年份，一週從週日開始，週六結束，只要本週跨年，那麼這周就算入下一年。

If a week is split at the end of the year then it is assigned to the year in which more that half of the days of that week occur.

u year
y year-of-era
Y week-based-year

public static void main(String[] args) {  
    DateFormat df = new SimpleDateFormat("yyyyMMdd");  
    System.out.println(df.format(new Date()));  
    DateFormat df1 = new SimpleDateFormat("YYYYMMdd");  
    System.out.println(df1.format(new Date()));  
    Calendar calendar = Calendar.getInstance();  
    // 2019-12-31  
    calendar.set(2019, Calendar.DECEMBER, 31);  
    Date strDate1 = calendar.getTime();  
    // 2020-01-01  
    calendar.set(2020, Calendar.JANUARY, 1);  
    Date strDate2 = calendar.getTime();  
    // 大寫 YYYY  DateFormat formatUpperCase = new SimpleDateFormat("YYYY/MM/dd");  
    System.out.println("2019-12-31 to YYYY/MM/dd: " \+ formatUpperCase.format(strDate1));  
    System.out.println("2020-01-01 to YYYY/MM/dd: " \+ formatUpperCase.format(strDate2));  
    // 小寫 YYYY  DateFormat formatLowerCase = new SimpleDateFormat("yyyy/MM/dd");  
    System.out.println("2019-12-31 to yyyy/MM/dd: " \+ formatLowerCase.format(strDate1));  
    System.out.println("2020-01-01 to yyyy/MM/dd: " \+ formatLowerCase.format(strDate2));  
}