Added polyfill (#25130)

Added polyfill for legacy browsers

guide/english/javascript/get-timestamp/index.md

@@ -8,3 +8,13 @@ You can easily convert the timestamp to seconds like this: `Math.floor(Date.now(
 If your browser does not support `Date.now()`, you can use `new Date().getTime()` to get the timestamp in milliseconds.
 
 Unix Timestamp is number of seconds elapsed since 01/01/1970, 00:00:00 UTC. The Unix timestamp will break on 01/19/2038.
+
+### Polyfill
+For older browsers, we can use:
+```js
+if (!Date.now) {
+  Date.now = function now() {
+    return new Date().getTime();
+  };
+}
+```