title: Determine the difference between two DateTimes, only counting opening hours
body: <p>For our support software in C#, I need to determine the time span between two DateTimes, but I only want opening hours counted (i.e. weekdays from 09:00 to 17:00).</p>
  
  <p>So, for instance, if the first DateTime is 15/02/2011 16:00 and the second is 16/02/2011 10:00, the method shall return 2 hours.</p>
  
  <p>Any help is greatly appreciated!</p>