title: How to tween a data.frame with date format timepoints?
body: <p>I would like to use <code>tweenr</code> with a <code>date</code> format variable for the <code>timepoints</code>.</p>
  
  <p>However, <code>tweenr::tween_elements</code> throws an error, that I cannot decipher:</p>
  
  <pre><code>&gt; Error in `/.difftime`(diff(timerange), nframes) : 
    second argument of / cannot be a "difftime" object
  </code></pre>
  
  <p>Am I using the wrong <code>tweenr</code> function? This is most probably some expected behaviour, but I can't get my head round this.</p>
  
  <p><em>Reproducible Example:</em></p>
  
  <pre><code>if (!require(devtools)) {
    install.packages("devtools")
  }
  devtools::install_github("thomasp85/tweenr")
  
  library(ggplot2)
  library(gganimate)
  library(ggforce)
  library(tweenr)
  
  data &lt;- data.frame(
    time = rep(seq(as.Date("2000-01-01"), 
                   as.Date("2009-01-01"),
                   "year"),
               2),
    x = c(1:10,20:11),
    y = c(20:11,1:10),
    group = c(rep(1,10), rep(2,10)),
    ease = rep('cubic-in-out', 20)
  )
  
  data &lt;- tween_elements(data, 'time', 'group', 'ease')
  </code></pre>