Test expired.in returns positive ms for valid cache

package.json

@@ -34,6 +34,7 @@
     "coveralls": "^2.11.15",
     "date-fns": "^1.21.1",
     "nyc": "^10.0.0",
+    "timekeeper": "^1.0.0",
     "xo": "^0.17.1"
   }
 }

test/expired.in.js

@@ -1,6 +1,21 @@
 import test from 'ava';
+import tk from 'timekeeper';
 import expired from '../';
 
 test('expired.in is a function', t => {
 	t.is(typeof expired.in, 'function');
 });
+
+test('expired.in returns positive ms for valid cache', t => {
+	const date = new Date().toUTCString();
+	const maxAge = 300;
+	const headers = {
+		date: date,
+		age: 0,
+		'cache-control': `public, max-age=${maxAge}`
+	};
+
+	tk.freeze(date);
+	t.is(expired.in(headers), maxAge * 1000);
+	tk.reset();
+});