[javascript] How to measure time elapsed on Javascript?

I created a simple game that start and ends the timer when the user finishes clicking on 16 boxes.

I want to measure the elapsed time for the user to complete the game. How do I do it using Javascript?

I took a look at different answers like this, but I had hard time understanding others' code.

I would assume it to look like this.

Timer Start: When user clicks the first box
Timer End: When user clicks the last box

var seconds = 0;
setInterval(function () {
  seconds++;
}, 1000);

There you go, now you have a variable counting seconds elapsed. Since I don't know the context, you'll have to decide whether you want to attach that variable to an object or make it global.

Set interval is simply a function that takes a function as it's first parameter and a number of milliseconds to repeat the function as it's second parameter.

You could also solve this by saving and comparing times.

EDIT: This answer will provide very inconsistent results due to things such as the event loop and the way browsers may choose to pause or delay processing when a page is in a background tab. I strongly recommend using the accepted answer.