How can I do a countdown timer with html?

I'm helping my I.T teacher to create easter eggs for some tasks, and I would like to create a countdown timer with html. Explanation: Everytime you enter into a website, the countdown timer starts. Example: I have a html code with a countdown timer at 30 min, if I go into the website, the countdown timer starts going down, but if I refresh the website, it reset. I hope you will understand, thanks!