title: R POSIXct returns NA with "03/12/2017 02:17:13"
body: <p>I have a data set containing the following date, along with several others</p>
  
  <pre><code>03/12/2017 02:17:13
  </code></pre>
  
  <p>I want to put the whole data set into a data table, so I used <code>read_csv</code> and <code>as.data.table</code> to create DT which contained the date/time information in date. 
  Next I used</p>
  
  <pre><code>DT[, date := as.POSIXct(date, format = "%m/%d/%Y %H:%M:%S")]
  </code></pre>
  
  <p>Everything looked fine except I had some NA values where the original data had dates. The following expression returns an NA</p>
  
  <pre><code>as.POSIXct("03/12/2017 02:17:13", format = "%m/%d/%Y %H:%M:%S")
  </code></pre>
  
  <p>The question is why and how to fix.</p>