title: How to get date difference linq
body: <p>I was trying to filter birthdays which comes within the next 14 days. No error was shown even on built. All data are presented to the <strong>result</strong>. But during run time it throws "InValidOperationException"(CS0119).</p>
  
  <pre><code>        if (filterModel.IsNextBirthday.HasValue)
          {
              var filteredResult = result.Where(x =&gt; x.People.SelectMany(s =&gt; s.Details).Any(m =&gt; 
                                   ((m.BirthDate).Subtract(DateTime.Today)).Days &lt; 14));
          }
  </code></pre>