title: Summarizing rows on unique value, except time column
body: <p>I have a thinking problem (not sure if there is already a solution on stack, but I have no idea how I can fix this). </p>
  
  <p>I have a data frame like this: </p>
  
  <pre><code>ID      Visits    Time                      X    Y    Z 
  1        2        2016-05-15 06:38:40       1    1    0 
  1        4        2016-05-15 07:38:40       0    0    1 
  1        2        2016-05-15 08:38:40       0    1    0 
  2        3        2016-05-15 09:38:40       1    0    2 
  3        2        2016-05-15 10:38:40       0    1    0 
  3        1        2016-05-15 11:38:40       1    0    1 
  </code></pre>
  
  <p>I want to make a new data frame, with:</p>
  
  <ul>
  <li>Unique ID's (I guess with a group_by() ); </li>
  <li>The other columns (except Time) summed; </li>
  <li>Keep the first Time </li>
  </ul>
  
  <p>So the result should be this: </p>
  
  <pre><code>ID      Visits    Time                      X    Y    Z 
  1        8        2016-05-15 06:38:40       1    2    1 
  2        3        2016-05-15 09:38:40       1    0    2 
  3        3        2016-05-15 10:38:40       1    1    1 
  </code></pre>
  
  <p>I tried this: </p>
  
  <pre><code>data %&gt;% group_by(ID) %&gt;% summarise_at(vars(-Time), funs(sum.,na.rm = TRUE)))
  </code></pre>
  
  <p>But, there is my thinking issue: the variable Time is now out of my data, and I can't add the variable anymore (because it is not the same length anymore).</p>