title: Temporal distance matrix from dates
body: <p>From a very simple dataframe like</p>
  
  <pre><code>    time1 &lt;- as.Date("2010/10/10")
      time2 &lt;- as.Date("2010/10/11")
      time3 &lt;- as.Date("2010/10/12")
      test &lt;- data.frame(Sample=c("A","B", "C"), Date=c(time1, time2, time3))
  </code></pre>
  
  <p>how can i obtain a matrix with pairwise temporal distances (elapsed time in days between samples) between the Samples A, B, C?</p>
  
  <pre><code>   A  B  C
  A  0  1  2
  B  1  0  1
  C  2  1  0
  </code></pre>
  
  <p>/edit: changed the format of the dates. sorry for inconveniences</p>