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。
