Animate beginners javascript function issue

Hi – I've just started with playing around with javascript in Animate (not sure it's even the right software for the animation I want to do yet!), but am having a beginners' issue with getting functions to run. I've adapted some of the code from this tutorial.

When I use this code in the first keyframe to change the text on a Dynamic Text box called 'yearText' it works fine:

var currentYear = 2017;

var year = 1971;

this.yearText.text = year;

this.playBtn.addEventListener("click", playClicked.bind(this));

function playClicked() {

    this.play();

    isPlaying = true;

    year = year + 1;

    this.yearText.text = year;

}

But then if I put a function inside the function in place of the last two lines like this, it no longer works:

this.playBtn.addEventListener("click", playClicked.bind(this));

function playClicked() {

    this.play();

    isPlaying = true;

    year = year + 1;

    this.yearText.text = year;

}

function yearIncrease() {

        year = year + 1;

        this.yearText.text = year;

    }

Have I completely misunderstood something about how functions work?

Any help very much appreciated! Then i can see if Animate is something I'll be able to use properly...