title: UNIX DATE: how to calculate a date
body: <p>I'm new at UNIX and I do some exercises and my goal is to get number as parameter - "n"
  than calculate the date of today less n</p>
  
  <pre><code>(TODAY -N = answer)
  </code></pre>
  
  <p>and the answer will be displayed as</p>
  
  <pre><code>date +%a-%b-%d-%Y
  </code></pre>
  
  <p>how do I do it in easy way?
  thanks in advance :)</p>