title: SSRS get sum on an expression field
body: <p>Suppose I have the following data:<br></p>
  
  <blockquote>
  <pre><code>TimeTable
  Person  Week   Date        EnterTime ExitTime PeriodDiff
  ----------------------------------------------------
  John    1      01.01.2018  09:15     10:35    1:20
  John    1      01.01.2018  10:55     12:23    1:28
  John    1      01.01.2018  13:00     17:35    4:35
  John    1      02.01.2018  09:00     16:35    7:35
  John    2      08.01.2018  09:05     11:40    2:35
  John    2      08.01.2018  16:15     19:35    3:20
  John    2      09.01.2018  10:50     21:57    11:07
  </code></pre>
  </blockquote>
  
  <p>I am trying to make a report about the weekly and daily time people were in the company.
  So I am trying to create the following report:</p>
  
  <blockquote>
  <pre><code>                 +[WeekGroup]                                                  +
                   ---------------------------------------------------------------
                                  +[DateGroup]                                   +
                   WeeklyTotal    ------------------------------------------------
  Person                          FirstEntry     LastExit      TimeInside   Period
  [PersonGroup]    &lt;&lt;Exp3&gt;&gt;       Min[EnterTime] Max[ExitTime] &lt;&lt;Exp1&gt;&gt;     &lt;&lt;Exp2&gt;&gt;   
  </code></pre>
  </blockquote>
  
  <p>Exp1 = Max(ExitTime) - Min(EnterTime) <br> 
  Exp2 = Sum(PeriodDiff)<br>
  Exp3 should be Sum(Max(ExitTime) - Min(EnterTime)) for each day<br></p>
  
  <p>Everything works except for Exp3.<br>
  Problem is Exp3 is outside of the DateGroup so the min/max values refer to each of those in the respected week. Also I cannot use the sum of period because of the gaps in time.<br>
  How can I get the sum of each weeks calculated time differance? Meaning sum(Exp1) ?</p>