title: The -d option format of the Bash command date
body: <p>In <code>date</code>'s man page：</p>
  
  <pre><code>-d, --date=STRING         display time described by STRING, not 'now'
  </code></pre>
  
  <p>Sounds like you should input a exact date, but I find <code>date -d "-1 day"</code> works. </p>
  
  <p>I just want a list of the format of <code>-d option</code> of the command <code>date</code>. I googled, but find none.</p>