Countdown timers previously added using adobe flash wont don't work in win10Windows 10
The following links are an provide alternative I think this one is the most popular … options:
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-a368-8b4ffc31a606And the TES website has lots of downloadable tools to help too.
https://www.tes.com/resources/search/?q=powerpoint%20timer (You need to create an account to use this)