title: Calculate datetime difference in automapper mapping (UseValue)
body: <p>I have situation where I need to know difference between two dates in days. I wanna do this in <code>mapper conf</code>. </p>
  
  <p>The problem is I dont know is this possible and how?</p>
  
  <p>Here is example code:</p>
  
  <pre><code>CreateMap&lt;Post, ManagePostViewModel&gt;()
  .ForMember(d =&gt; d.ActiveInDays, conf =&gt; conf.UseValue((DateTime.UtcNow - (conf.CreatedAt)).TotalDays));
  </code></pre>
  
  <p>I tested this syntax in static values and it works. If I use <code>conf.CreatedAt</code> it dosn't, gives following error:</p>
  
  <blockquote>
    <p>'IMemberConfigurationExpression' does not contain definition for 'CreatedAt' and no extension method 'CreatedAt' accepting a first argument of type 'IMemberConfigurationExpression' could be found (are you missing a using directive or an assembly reference?) </p>
  </blockquote>
  
  <p>In <code>model</code> and <code>viewModel</code> i have defined <code>CreatedAt</code>.</p>