title: converting time stamp to minutes elapsed taking calendar start as reference in R
body: <p>have some time stamp data</p>
  
  <pre><code>2016-03-01 21:02:56.440
  </code></pre>
  
  <p>can this be converted to number of minutes taking start of calendar as reference</p>
  
  <pre><code>1456846376.44
  </code></pre>
  
  <p>units is minutes
  any leads will be appreciated</p>