title: strptime in R returning NA
body: <p>I have a variable in my dataset called timestamp which is of the form.
  mydata$timestamp</p>
  
  <p><code>2013-08-01 12:00:00 
  2013-08-01 12:00:00 
  2013-08-01 12:00:00</code></p>
  
  <p>I want to modify these and change them to only dd-mm-yy format
   <code>dates&lt;-strptime(mydata$timestamp, format="%d:%m:%y")</code>
   printing dates is resulting in only NA's.  Not sure why.  </p>
  
  <p>Could anyone help, please?</p>
  
  <p>Thanks in advance</p>