title: Extracting time from POSIXct
body: <p>How would I extract the time from a series of <code>POSIXct</code> objects discarding the date part?</p>
  <p>For instance, I have:</p>
  <pre><code>times &lt;- structure(c(1331086009.50098, 1331091427.42461, 1331252565.99979, 
  1331252675.81601, 1331262597.72474, 1331262641.11786, 1331269557.4059, 
  1331278779.26727, 1331448476.96126, 1331452596.13806), class = c(&quot;POSIXct&quot;, 
  &quot;POSIXt&quot;))
  </code></pre>
  <p>which corresponds to these dates:</p>
  <pre><code>&quot;2012-03-07 03:06:49 CET&quot; &quot;2012-03-07 04:37:07 CET&quot; 
  &quot;2012-03-09 01:22:45 CET&quot; &quot;2012-03-09 01:24:35 CET&quot; 
  &quot;2012-03-09 04:09:57 CET&quot; &quot;2012-03-09 04:10:41 CET&quot;
  &quot;2012-03-09 06:05:57 CET&quot; &quot;2012-03-09 08:39:39 CET&quot;
  &quot;2012-03-11 07:47:56 CET&quot; &quot;2012-03-11 08:56:36 CET&quot;
  </code></pre>
  <p>Now, I have some values for a parameter measured at those times:</p>
  <pre><code>val &lt;- c(1.25343125e-05, 0.00022890575, 
           3.9269125e-05, 0.0002285681875, 
           4.26353125e-05, 5.982625e-05, 
           2.09575e-05, 0.0001516951251, 
           2.653125e-05, 0.0001021391875)
  </code></pre>
  <p>I would like to plot val vs time of the day, irrespectively of the specific day when val was measured.</p>
  <p>Is there a specific function that would allow me to do that?</p>