title: as.POSIXct with datetimes including midnight
body: <p>I am wanting to convert date-times stored as characters to date-time objects.
  However if a date time includes midnight then the resulting datetime object excludes the time component, which then throws an error when used in a later function (not needed here - but a function that extracts weather data for specified location and date-time).</p>
  
  <p>Example code:</p>
  
  <pre><code>example.dates &lt;- c("2011-11-02 00:31:00","2011-11-02 00:00:00","2011-11-02 00:20:22")
  posix.dates   &lt;- as.POSIXct(example.dates, tz="GMT", format="%Y-%m-%d %H:%M:%S")
  posix.dates
  posix.dates[2]
  </code></pre>
  
  <p>NB times is only excluded when the datetime containing midnight is called on it's own (atomic vector).</p>
  
  <p>Is there a way of retaining the time data for midnight times? Can you suggest an alternative function?</p>