title: Unix time clarification
body: <p>So, this has sortof already been asked: <a href="https://stackoverflow.com/questions/16539436/unix-time-and-leap-seconds">Unix time and leap seconds</a></p>
  
  <p>...but I'm just not getting it.  Judging from the lengthy and varied comments, I'm not convinced anybody else does, either.</p>
  
  <p>So, I'll ask my own question.  Consider that there is an exact value of a "real" second: <a href="https://en.wikipedia.org/wiki/Second" rel="nofollow noreferrer">9192631770 transitions of a caesium 133 atom</a>.  As I understand, this does not align perfectly with our idea of 60*60*24 seconds in a day, because the length of a day varies, basically, and THOSE don't line up perfectly with year boundaries, hence leap days and leap seconds, etc.  Now, suppose that on 00:00:00 Thursday, 1 January 1970, I started a caesium clock going.  According to <a href="https://currentmillis.com" rel="nofollow noreferrer">currentmillis.com</a>, as I write this, Unix time is 1565994103 seconds.</p>
  
  <p>Would the caesium clock agree?</p>