日期格式化 YYYY日期错位

起因

支付系统通知账务还款结果时,对请求时间使用“YYYYMMddHHmmss”格式化,20171231变为了20181231致使数据不匹配。

我们大多都知道mm与MM的区别,因为分和月会同时出现,但很少会关注其它的pattern大小写有无区分。

重现demo如下

Date date = new Date(2017 - 1900, 11, 31);

DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
System.out.println(dateFormat.format(date));

dateFormat = new SimpleDateFormat("YYYY-MM-dd");
System.out.println(dateFormat.format(date));

输出结果:
2017-12-31
2018-12-31

Date and Time Patterns

Letter Date or Time Component Presentation Examples
G Era designator Text AD
y Year Year 1996; 96
Y Week year Year 2009; 09
M Month in year (context sensitive) Month July; Jul; 07
L Month in year (standalone form) Month July; Jul; 07
w Week in year Number 27
W Week in month Number 2
D Day in year Number 189
d Day in month Number 10
F Day of week in month Number 2
E Day name in week Text Tuesday; Tue
u Day number of week (1 = Monday, …, 7 = Sunday) Number 1
a Am/pm marker Text PM
H Hour in day (0-23) Number 0
k Hour in day (1-24) Number 24
K Hour in am/pm (0-11) Number 0
h Hour in am/pm (1-12) Number 12
m Minute in hour Number 30
s Second in minute Number 55
S Millisecond Number 978
z Time zone General time zone Pacific Standard Time; PST; GMT-08:00
Z Time zone RFC 822 time zone -0800
X Time zone ISO 8601 time zone -08; -0800; -08:00

以上y为正常理解中的year,Y为week year,当天所在的周所属年份,一周从周日开始,周六结束。只要本周跨年,那么这周即算入下一年,2017-12-31为周日,刚好是2017最后一周并跨年。用YYYY获取自然得出2018,造成了这个错误。

参考

SimpleDateFormat by Java
Date Formatters by iOS

转载请标明出处:
http://2tu.github.io/2018/01/03/date-formate-YYYY-in-java/
本文出自Tu’s blog