title: Nginx Cache: how to expire/vary by request date
body: <p>I've got content that changes depending on the date it is requested on, i.e. 
  the same content will be served when the page is requested at <code>2018-09-10T00:00</code> and <code>2018-09-10T23:59</code>, but different content will be served at <code>2018-09-11T00:00</code> (based on the server time).</p>
  
  <p>Is there a <code>vary</code> http header I can set on the content that will achieve this (set by the upstream server)?</p>
  
  <p>Or is there another way of doing this in nginx (ideally only on certain paths)?  </p>