Test expired returns true for stale cache

package.json

@@ -32,6 +32,7 @@
   "devDependencies": {
     "ava": "^0.17.0",
     "coveralls": "^2.11.15",
+    "date-fns": "^1.21.1",
     "nyc": "^10.0.0",
     "xo": "^0.17.1"
   }

test/expired.js

@@ -1,4 +1,5 @@
 import test from 'ava';
+import subSeconds from 'date-fns/sub_seconds';
 import expired from '../';
 
 test('expired is a function', t => {
@@ -14,3 +15,13 @@ test('expired returns false for valid cache', t => {
 
 	t.false(expired(headers));
 });
+
+test('expired returns true for stale cache', t => {
+	const headers = {
+		date: subSeconds(new Date().toUTCString(), 500),
+		age: 0,
+		'cache-control': 'public, max-age=300'
+	};
+
+	t.true(expired(headers));
+});