Drupal 8 Date widget help text for browsers that dont support html5 date input.

Drupal-8-Date-Help-Text-JS

  // HTML5 date elements use the user's browser date format,
  // but browsers that don't support it have a more confusing interface
  // requiring help text. 
  Drupal.behaviors.DateHelp = {
    attach: function (context, settings) {
      if (Modernizr.inputtypes.date === false) {
        // If there is no help text div, first add one.
        $('div.field--widget-datetime-default:not(:has(.description))').each(function() {
          $(this).find('input').after('<div class="description"></div>');
        });
        $('.field--widget-datetime-default .description').append(' Enter dates as yyyy-mm-dd.');
      }
    }
  }