title: Difference between 'yy' and 'YY' in Java Time Pattern
body: <p>From document <a href="https://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html" rel="noreferrer">SimpleDateTimePattern</a>, <code>yy</code> should be the same with <code>YY</code>.</p>
  
  <p>Today is <code>Dec 30, 2019</code>, now we get <code>YY</code> for today is <code>20</code>, <code>yy</code> for today is <code>19</code>. What's the difference between <code>yy</code> and <code>YY</code> in Java Time Pattern?  </p>