title: Converting time format to numeric with R
body: <p>In most cases, we convert numeric time to POSIXct format using R. However, if we want to compare two time points, then we would prefer the numeric time format. For example, I have a date format like &quot;2001-03-13 10:31:00&quot;,</p>
  <pre><code>  begin &lt;- &quot;2001-03-13 10:31:00&quot;
  </code></pre>
  <p>Using R, I want to covert this into a numeric (e.g., the Julian time), perhaps something like the passing seconds between 1970-01-01 00:00:00 and 2001-03-13 10:31:00.</p>
  <p>Do you have any suggestions?</p>
  <hr />
  <p>The Julian calendar began in 45 BC (709 AUC) as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year (known at least since Hipparchus). see <a href="http://en.wikipedia.org/wiki/Julian_calendar" rel="nofollow noreferrer">http://en.wikipedia.org/wiki/Julian_calendar</a></p>