Two things (I suppose) are special about it:

- The timers are “programmable”, so you can freely express your own routines and procedures in a declarative notation.

- The app is all static (no backend): the entire program is encoded in the URL and can be bookmarked or shared/transferred via QR-code.

You can check it out at https://timer.jotaen.net, optionally with a demo program pre-loaded: https://timer.jotaen.net/#demo.

Source code is at https://github.com/jotaen/timer. I’ve also written up a small behind-the-scenes on my blog: https://www.jotaen.net/SAKxq