The timestamp read-only property of the Notification interface returns a number, as specified in the timestamp option of the Notification() constructor.

The notification's timestamp can represent the time, in milliseconds since 00:00:00 UTC on 1 January 1970, of the event for which the notification was created, or it can be an arbitrary timestamp that you want associated with the notification. For example, a timestamp for an upcoming meeting could be set in the future, whereas a timestamp for a missed message could be set in the past.

A number representing a timestamp, given as Unix time in milliseconds.

The following snippet fires a notification; a simple options object is created, then the notification is fired using the Notification() constructor.

const dts = Math.floor(Date.now());

const options = {
  body: "Your code submission has received 3 new review comments.",
  timestamp: dts,
};

const n = new Notification("New review activity", options);

console.log(n.timestamp); // Logs the timestamp

• Using the Notifications API