Install Python (https://www.python.org).
Double click countdown.py.
If that didn't work, open a command prompt and run python countdown.py.
Type in the amount of time you want to wait for, press "Set" to set the time, and press "Start" to start the timer.
Example times: 30 for 30 seconds, 12:30 for 12 minutes and 30 seconds, 1:20:30 for an ohour, 20 minutes, and 30 seconds.
Press "Stop" to stop the timer, and press "Reset" to clear the time.