title: character string is not in a standard unambiguous format
body: <p>I have a dataframe (df3) with with some values.</p>
  
  <p>One of these values is the daedlines.</p>
  
  <p>The data of this value is something like the following:</p>
  
  <pre><code>deadline
  1419397140
  1418994978
  1419984000
  1418702400
  </code></pre>
  
  <p>They are days and I want to convert the to using this:</p>
  
  <pre><code>df3$deadline &lt;- as.POSIXct(df3$deadline, origin="1970-01-01")
  </code></pre>
  
  <p>Generally it was worked for me with other dataframes from other files.</p>
  
  <p>However with this it gives me back this error:</p>
  
  <pre><code>Error in as.POSIXlt.character(as.character(x), ...) : 
    character string is not in a standard unambiguous format
  </code></pre>
  
  <p>How can I fix it?</p>