title: Convert date column to unix date column of a dataframe
body: <p>Using this it gives the date time format to the specific column </p>
  
  <pre><code> df[,":="(date = as.numeric(as.POSIXct(date , "%Y-%m-%d %H:%M:%S"))]
  </code></pre>
  
  <p>How is it possible to convert a date column to a epoch/unix date column like this it can be for one value</p>
  
  <pre><code>as.numeric(as.POSIXct("2013-09-16 2:13:46 EST"))
  </code></pre>