title: How do i get last sunday date from a given date in unix
body: <p>get last sunday date as output from a given date (not current date) as input</p>
  
  <p>Example input: 08-30-2017 (%m-%d-%Y)</p>
  
  <p>output should be last sunday: 08-27-2017</p>
  
  <p>===</p>
  
  <p>All of the below commands use current day as reference .But i want to give the reference date as input to get last sunday. Please help me with the command.</p>
  
  <p>date +%m-%d-%Y -d "2017-09-10 -7 days"</p>
  
  <p>date +%m-%d-%Y -d "last Sun"</p>