This is a simple countdown in plain HTML+CSS+JavaScript. You can either host it yourself (just serve the files in this repository with a web server) or use https://goodtimer.lubiland.de
The target date is specified behind a hash (#) and in any [ECMAScript Date Time String Format](https://tc39.es/ecma262/#sec-date-time-string-format). When you omit the timezone it uses the local time of the browser.
Goodtimer is also usable as widget for [Matrix](https://matrix.org) clients. Within [Element](https://element.io) you can either specify the URL as 'Custom Widget' or manually send something like this example state event via `/devtools`: