title: ymd with vector of dates
body: <p>A simple question, I think.  I have some dates, <code>d</code>: </p>
  
  <pre><code>d &lt;- as.POSIXct(c("2014-01-01 00:00:00 BST", "2014-01-01 00:30:00 BST"))
  &gt; class(d)
  [1] "POSIXct" "POSIXt" 
  </code></pre>
  
  <p>If I try and extract just the date part with <code>lubridate</code>, it works fine with a single value but not the whole vector, i.e.:</p>
  
  <pre><code>&gt; ymd(d[1])
  [1] "2014-01-01 UTC"
  
  &gt; ymd(d)
  [1] NA NA
  Warning message:
  All formats failed to parse. No formats found.
  </code></pre>
  
  <p>For the record, this works:</p>
  
  <pre><code>&gt; as.Date(d, format="%F")
  [1] "2014-01-01" "2014-01-01"
  </code></pre>
  
  <p>What's going on here?</p>