title: Why does anytime return NA in some cases
body: <p>Why does anytime return NA in the first case but a valid result in the second case.</p>
  
  <pre><code> str(A$X1)
   int [1:23744] 1487319525 1487323025 1487325865 1487332405 1487334780 1487353675 1487354135 1487354185 1487354195 1487354215 ...
  &gt; anytime(A$X1[1])
  [1] NA
  &gt; anytime(1487319525)
  [1] "2017-02-17 02:18:45 CST"
  </code></pre>