header: fix scope problem

templates/partials/header.html

@@ -27,6 +27,7 @@
 </navbar>
 
 <script>
+(function() {
   const dateEl = document.querySelector('current-date');
   if (dateEl) {
     const update = () => {
@@ -39,4 +40,5 @@
     update();
     setInterval(update, 1000);
   }
+})();
 </script>