title: How to get seconds from boost::posix_time::ptime
body: <p>First off: I'm a very new to Boost and somewhat new to C++, too. 
  I',m working with a  function where I get the following <code>boost::posix_time::ptime time</code> passed as an argument and I want to extract the <code>POSIX</code> time aka "seconds since epoch" from this as <code>long</code>. How do I do this?</p>