title: Convert date to unix timestamp in AIX
body: <p>I have few dates which are converted like that year month &amp; date:</p>
  
  <pre><code>20151213 20151214
  </code></pre>
  
  <p>In AIX seems <code>date -d</code> not recognize -d option</p>
  
  <pre><code>date -d 20151213 +%s
  </code></pre>
  
  <p>So I'm not able to convert that specific date format which I receive dynamically to Unix time stamp.<br>
  How to convert those dates in AIX to timestamp?</p>