学艺不精，以为YYYY和yyyy可以一样用，结果翻车了……
根据Oracle Java SE 7的文档，可以知道小写y指的是年，大写Y指的是Week Year。但是Week Year是什么东东？
开始无脑引用（参考资料1）：
The week of a week-based year can also be defined in different ways.
- Week # 1 contains January 1.
- Week # 1 is the first week to contain a particular day-of-week such as Sunday.
MD，看了一圈下来发现：Week Year基于周的定义来计算年，且在标准Java环境下，Week Year在和Week in year(Letter w)和Day number of week(Letter u)联用的情况下才能正常工作。在Y与别的Letter混用的情况下，别的值不能正常使用，值会被认为是1。（Since you supplied a week-year but did not supply day of week and week in year, those to values have been assumed as 1.）
不过在Pega环境下，Y似乎不按照Java基本法发挥作用，“YYYY-MM-DD”居然会被认为是“自然年-月-Day in year”，太神奇了。