The clock shows the local time in one city. It reads the visitor's device clock and the IANA zone for that city, so the time stays current after the page that hosts it was built.
The clock-change countdown tells you how long until the clocks next change in a country or a city, and the local time of that change. When the change passes, the count moves to the next one. A place with no clock change says so and links to its time page.
Example: <iframe src="https://worlddateandtime.com/embed/clock?city=united-kingdom/london" title="Local time in London" width="320" height="200" loading="lazy"></iframe>