By default, Web Notifications on Android display the time at which they were created in their top-right corner. However, this is not always appropriate. For example, when the device was offline the event could have happened in the past, or the notification could be shown for an upcoming meeting. The `timestamp` property enables developers to provide the time. Note that we don't visually surface the timestamp on desktop right now, but do use it to affect notification ordering.
Docs: https://developers.google.com/web/updates/2016/03/notifications
Samples: https://tests.peter.sh/notification-generator/#timestamp=2