title: Weeks between two dates in Postgres
body: <p>I want to calculate the number of weeks that a customer has been a customer. The goal is to compare this to the distinct number of weeks the customer has made a purchase to determine what % of weeks the customer made a purchase.</p>
  
  <p>With timestamp <code>customer.created_at</code> what is the best way to find the number of weeks a customer has been a customer? Said differently, the difference in weeks between the current week and the week in which the customer signed up.</p>
  
  <p>Calendar weeks starting Monday to be apples-to-apples with distinct calendar weeks a customer has made a purchase.</p>