title: Problems with dplyr and POSIXlt data
body: <p>I have a problem. I downloaded data and tranformed dates into POSIXlt format</p>
  
  <pre><code>df&lt;-read.csv("007.csv", header=T, sep=";")
  df$transaction_date&lt;-strptime(df$transaction_date, "%d.%m.%Y")
  df$install_date&lt;-strptime(df$install_date, "%d.%m.%Y")
  df$days&lt;- as.numeric(difftime(df$transaction_date,df$install_date, units = "days"))
  </code></pre>
  
  <p>Data frame is about transaction in one online game. It contains value (its payment), transaction_date, intall_date and ID. I added new column, which showndays after installation. I tried to summarise data using dlyr</p>
  
  <p><code>df2&lt;-df %&gt;%
    group_by(days) %&gt;%
    summarise(sum=sum(value))</code></p>
  
  <p>And I've got an error:
  Error: column 'transaction_date' has unsupported type : POSIXlt, POSIXt</p>
  
  <p>How can i Fix it?</p>
  
  <p>UPD. I changed classes of Date columns into Character. It solved problem. But can i use dlyr withouts changing classes in my dataset?</p>