title: How to apply summarise_each to all columns except one?
body: <p>I am analyzing a set of data with many columns (almost 30 columns). I want to group data based on two columns and apply sum and mean functions to all the columns except timestamp. 
  How would I use summarise_each on all columns except timestamp?</p>
  
  <p>This is the draft code I have but it obviously not correct. Plus it generates and error because it can not apply Sum to POSIXt data type (Error: 'sum' not defined for "POSIXt" objects) </p>
  
  <pre><code>features &lt;- dataset %&gt;% 
    group_by(X, Y) %&gt;% 
    summarise_each(funs(mean,sum)) %&gt;%
    arrange(TIMESTAMP)
  </code></pre>