title: POSIXct from character returns NA
body: <p>I have a time in excel that when converter to R, comes as a character and looks someting like this <code>0.59658.</code> </p>
  
  <p>I am trying to convert to POSIXct but it returns as a POSIXct with NA.</p>
  
  <pre><code>teste &lt;- as.POSIXct(test, format = "%H:%M")
  </code></pre>
  
  <p>I've also tried <code>teste &lt;- as.POSIXct(test, format = "%H:%M:%S")</code>
  For other columns it works fine, but not this one..</p>
  
  <p>UPDATE: 
  I've done the solution, but a second problem comes with the rest of the thing that I need. </p>
  
  <pre><code>teste &lt;- as.POSIXct(teste*24*60*60,"%H%M", origin="1970-01-01") 
  teste &lt;- format(as.POSIXct(teste, format = "%Y-%m-%d %H:%M:%S"), format="%H:%M")
  </code></pre>
  
  <p>And now, I want to paste with a date vector that is a POSIXct in the 2013-01-06, with this command:</p>
  
  <pre><code>teste&lt;-as.POSIXct(paste(date, teste), format="%Y-%m-%d %H:%M:%S")
  </code></pre>
  
  <p>And the NA are back</p>