Countdown timer with flip animation using css flexbox and vanilla js

Frontendmentor really teach me a lot. On every challenge, there are always something new to learn. I got some difficulities while doing this challenge to make the flip animation. Luckily, I found a youtube videos that shows me the concept behind it. And I learn a lot, at first it was really hard, but at the end, I can understand what is going on. But I know it's still not perfect. So any feedback or suggestions will be really appreciated.