Warn if parsing the date from the UTC timing element fails (#1318)

Backported to v2.3.x

Closes #1317

lib/dash/dash_parser.js

@@ -1420,15 +1420,23 @@ shaka.dash.DashParser.prototype.requestForTiming_ =
       .then(function(response) {
         var text;
         if (method == 'HEAD') {
-          if (!response.headers || !response.headers['date']) return 0;
+          if (!response.headers || !response.headers['date']) {
+            shaka.log.warning('UTC timing response is missing',
+                              'expected date header');
+            return 0;
+          }
 
           text = response.headers['date'];
         } else {
           text = shaka.util.StringUtils.fromUTF8(response.data);
         }
 
         var date = Date.parse(text);
-        return isNaN(date) ? 0 : (date - Date.now());
+        if (isNaN(date)) {
+          shaka.log.warning('Unable to parse date from UTC timing response');
+          return 0;
+        }
+        return (date - Date.now());
       });
 };