itsunny01-code/CountDown-Timer ? reverse-engineered prompt

Build me a simple countdown timer web page in plain HTML, CSS, and JavaScript.

I want a clean page where I can enter a future date or time, start the countdown, and see the remaining days, hours, minutes, and seconds update live on the screen. When the timer reaches zero, it should clearly show that the countdown is finished. Keep the design neat and easy to use, with a responsive layout that still looks good on mobile.

Please make it feel polished, with good spacing, readable text, and a simple visual style. If you need to check anything, look up current browser friendly approaches online.