title: difftime in R generate NA values
body: <pre><code>as.numeric(difftime(
  strptime("12:00 AM", "%I:%M %p" ),
  strptime("2:00 AM", "%I:%M %p" ),
   units='hours'))
  </code></pre>
  
  <p>result: NA.<br>
  I am trying to calculate the time difference, but some rows generate NA values which are really annoying, something wrong here?</p>