title: Calculate previous week's start and end date
body: <p>What is the best way to calculate the previous week's start and end date in C#?  I.e. today 18 March would result in 9 March (Monday last week) and 15 March (Sunday last week).</p>
  
  <p>I have seen this done with DayOfWeek and a switch statement to work out an offset but was wondering whether there is a more elegant way.</p>