title: R Shiny Date range input
body: <p>I have a date range input function as follows in my ui for my shiny app. </p>
  
  <pre><code>  dateRangeInput("dates", 
      "Date range",
      start = "2015-01-01", 
      end = as.character(Sys.Date()))
  </code></pre>
  
  <p>However I want a pop up message to correct the user if the user chooses a start date that is later than the end date, instead of an error in the app. How do i do this?</p>
  
  <p>Also is it possible to only allow the user to choose a date range which is more than, say x, days.</p>