robust date filter

filters.js

@@ -23,7 +23,7 @@ var filters = {
     'ceil': v => Math.ceil(v),
     'date': (v, arg) => {
         if (v === 'now') v = new Date();
-        return strftime(v, arg);
+        return v instanceof Date ? strftime(v, arg) : '';
     },
     'default': (v, arg) => v || arg,
     'divided_by': (v, arg) => Math.floor(v / arg),

test/filters.js

@@ -51,6 +51,9 @@ describe('filters', function() {
         it('should create a new Date when given "now"', function() {
             return test('{{ "now" | date: "%Y"}}', (new Date).getFullYear().toString());
         });
+        it('should render as empty string when invalid', function() {
+            return test('{{ "" | date: "%Y"}}', "");
+        });
     });
 
     describe('default', function() {