Hi @Justyna,
At the moment, Klaviyo doesn’t have a native countdown timer for emails yet, though I see it in the Signup Forms so maybe it will make it email templates one day.
What I’ve been using with my clients is a few third party ones. There’s many to choose from, but personally the one I always goto is Sendtric.com. They make it really easy to create a countdown timer, whether it’s a fixed timer or a dynamic one. I have no affiliation with tme, but you can check it out and see if it works for you.
Hope that helps!