title: R dates "origin" must be supplied
body: <p>My code:</p>
  <pre><code>axis.Date(1,sites$date, origin=&quot;1970-01-01&quot;)
  </code></pre>
  <p>Error:</p>
  <blockquote>
  <p>Error in as.Date.numeric(x) : 'origin' must be supplied</p>
  </blockquote>
  <p>Why is it asking me for the origin when I supplied it in the above code?</p>