title: Solr average of function result
body: <p>I have a solr index with documents that have a fields <code>started</code> and <code>stopped</code> which both hold a datetime. I would like solr to output an average difference between them. </p>
  
  <p>To get the difference between <code>started</code> and <code>stopped</code> I have used <code>diff:ms(started, stopped)</code> within fl.</p>
  
  <p>I know you can get stats about a field with <code>stats=true</code> and <code>stats.field=fieldname</code> but if i use either <code>diff</code> or <code>ms(started, stopped)</code> as the field name it errors with an undefined field error.</p>
  
  <p>So is what I want possible? If so, how do i go about it?</p>