Fix prop type for date in Countdown component.

src/Countdown.jsx

@@ -8,7 +8,7 @@ import style from './Countdown.sass';
 
 export default class Countdown extends React.Component {
   static propTypes = {
-    date: PropTypes.object.isRequired,
+    date: PropTypes.instanceOf(moment).isRequired,
     locale: PropTypes.string,
     minimumIntegerDigits: PropTypes.number,
     useGrouping: PropTypes.bool,