title: xaringan - can I set a countdown for the whole presentation?
body: <p>I know I can set a <code>countdown</code> for each slide with</p>
  <pre class="lang-r prettyprint-override"><code>---
  output:
    xaringan::moon_reader:
      nature:
        countdown: 60000
  ---
  </code></pre>
  <p>But what if I want to time my whole presentation? I mean, I have 15 minutes to present all slides.</p>