title: Why is NSTimeIntervalSince1970 defined as a fixed double?
body: <p>Just found this in <code>NSDate.h</code>:</p>
  
  <pre><code>#define NSTimeIntervalSince1970  978307200.0
  </code></pre>
  
  <p>It seems to be the UNIX timestamp of 01.01.2001 00:00:00 GMT.</p>
  
  <p>What is it for and why is it a fixed number?</p>