title: unlist a column with a list of dates in dplyr
body: <p>I've been looking at using the unnest() function in tidyr for a column that has lists of dates.</p>
  
  <pre><code>x &lt;- seq(from= as.POSIXct('2011-01-01 14:00:00'),length.out=100,by = "hour")
  
  y &lt;- seq(from= as.POSIXct('2012-01-01 14:00:00'),length.out=100,by = "hour")
  df &lt;- data.frame(x,y)
  </code></pre>
  
  <p>When I try to create a list for each row, and then unnest it. I get the following error. </p>
  
  <pre><code>df %&gt;% rowwise() %&gt;% mutate(sequence = list(seq.POSIXt(x,y,"10 min"))) %&gt;% unnest(sequence)
  </code></pre>
  
  <blockquote>
    <p>Error: Each column must either be a list of vectors or a list of data frames [sequence]</p>
  </blockquote>
  
  <p>Can someone else help with this? I've done this with numbers, and the unnest function works fine. However, it doesn't seem to work with lists containing date/ datetime. </p>