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I think this one is the most popular … https://www.online-stopwatch.com/countdown-timer/
If they want PowerPoint slides to do it, there is some help on the Microsoft website … https://support.office.com/en-us/article/insert-an-on-screen-timer-4c3a70d3-8d59-4972-a368-8b4ffc31a606
And the TES website has lots of downloadable tools to help too.