While you could use the java.util.Timer class, that uses multiple threads, which won't work with the HTML-deployment target, so you're better off sticking to the LibGDX library options.
The simplest option is just to use the TimeUtils class to record the time the game starts (in the constructor), and then measure how much time has passed (in the render() method, which gets called repeatedly by the game engine).
However, you could also look into LibGDX's Timer class which offers more sophisticated functionality.
In general, you may as well get used to researching answers for how to do things in LibGDX... e.g. by Googling "LibGDX timer count down" , and checking out the first few links to see if you can find something useful. That's a big part of real-world software development, actually...