There is already a world clock component for HA. This has to be included in the configuration.yaml and has limited options. It also has to be displayed in some way in Lovelace.
I missed a card that could be set up in ui-lovelace.yaml directly, without the need to restart HA, and with more options to configure the time as I wanted. This World Clock custom card is the result. In the example below, several instances of world-clock are arranged in three rows with horizontal-stack
.
Read more about it here: