nag-o-matic

Chrome extension

nag-o-matic is a free Chrome extension for named timers. Set a timer by duration or by date and time, and it nags you with a Chrome notification, an in-page overlay, or both when time's up. If Chrome was closed when a timer fell due, it alerts you on the next launch and shows how late you are.

The extension is free to use. Its source code is not published.