Datetime.app
World clock, UTC time, and timezone converter you can run in one Docker container.
Datetime.app shows the exact current time, UTC, and the time in cities around the world on one clean page, with a timezone converter and developer details such as Unix timestamps and ISO 8601 strings. People self-host it when they want a time.is style clock on their own domain or intranet, with analytics and ads switched off unless they turn them on.
The tradeoff is scope: it is a stateless web app, not a time server, so it does not replace NTP on your machines. The clock accuracy check also calls the public WorldTimeAPI from each browser, which means a fully offline network will lose that one feature.