As @lsw-fabian said, we have an example in the time tracker app, which shows the amount of elapsed seconds since the start time.
With frontend functions you can do almost anything data related. You are a bit limited by how you can present things like timers though.
You could write the current stopwatch value to the title of an input field (like in the example in the time tracker) or you could have an input field for a time attribute, which value you update.
If you want to be fancy, you could also use a readonly richtext field and update its value with a function - then you could use larger fonts:
Just a few options to work with. Of course you would probably still want buttons to start/stop timers and to store split times. All possible with frontend functions.