title: POSIXct ignore origin parameter
body: <p>This command in R</p>
  
  <pre><code>format(as.POSIXct(15*60,origin="2016-05-31 00:00:00",ts="UTC"),"%Y-%m-%d %H:%M")
  </code></pre>
  
  <p>gives me this</p>
  
  <pre><code>"2016-05-30 20:15"
  </code></pre>
  
  <p>Weirdly enough this:</p>
  
  <pre><code>format(as.POSIXct(15*60,origin="2016-05-31 00:00:00",ts="UTC","%Y-%m-%d %H:%M"))
  </code></pre>
  
  <p>gives me the expected result:</p>
  
  <pre><code>"2016-05-31 00:15:00"
  </code></pre>
  
  <p>and a warning message because the last bracket is not correctly placed.</p>
  
  <p>Why isn't my first command giving me the expected result (i.e "2016-05-31 00:15")? </p>