The timeline is all populated through a Google Spreadsheet. You can go to the website and it will download the spreadsheet for you to update. Once you embed the document, you can just update the spreadsheet and it will continue to update the timeline upon refresh.
https://timeline.knightlab.com/#make
Sample:
http://world.time.com/2013/12/05/nelson-mandelas-extraordinary-life-an-interactive-timeline/