title: How to find Previous Sunday in R
body: <p>It seems the Internet has not answered this question for R yet:</p>
  
  <p>If I have a date. Say the 20th of march: as.Date("2015-03-20")
  how do I get, in R, the previous Sunday?
  i.e., in the above example, as.Date("2015-03-15").</p>