You use your mobile phone or tablet as remote control and connect it to one screen that displays a countdown as selected on your remote control.
To get this working, you point one device or browser to this entry point as remote control and then another device or browser to the other entry point as display.
For instance, if you view this page currently from a desktop browser, then proceed with this desktop browser as your display device that will display a countdown timer. On the following pairing page, a QR-code is displayed that you can scan from your mobile phone, this is going to quickly put your mobile phone into remote control mode.
By following above links you, as a visiting user, are possibly already subject to terms and conditions (if legally applicable) even without creating a formal account: