title: Get current system time in milliseconds
body: <p>In Ruby, what is the right way to get the current system time since epoch(1970) in <strong>milliseconds</strong>?</p>
  
  <p>I tried <code>Time.now.to_i</code> , it seems not the result I want. I need the result shows milliseconds and with <code>long</code> type, not <code>float</code> or <code>double</code>.</p>