Another one with the same issue!!!

I have implemented this as a separate countdown card instead of hijacking the built in entity card (since this seems to be how must of the cards are done, instead of adding options to existing cards just add another card)

@piitaya I'm very happy to create a pull request if this is something that the maintainers might accept, I've seen PRs say that they should have been discussions before prs

mushroom-countdown-card
Just renders the entity state as a countdown (hours minutes seconds) hiding empty values as long as the state domain is timestamp. Otherwise behaves likes an entity card.

https://github.com/RickeyWard/lovelace-mushroom/tree/feature-countdown-timer-card

Working with maintainers on how to get this feature included, they don't like the idea of making it its own card but they haven't gotten back to me if they want additional options for base entity card.

In the meantime I created an addon build that adds the features for anyone that wants it.

Mushroom-countdown-card gist

IS there any progress on this? The JS file of @RickeyWard won't let you use a timer. entity.