Adding numbers (time) to a countdown

I’m sure this is something really basic, but I’ve searched and nothing quite fits what I’m looking for…

I have a timer counting down using this code

count = 30;
countdown = function(){
if (count ==0){

doCountdown = setInterval(countdown,1000);

and I want to be able to add 10 ‘seconds’ to it, when a button is pressed,
I tried

on (press) {

but this doesn’t work, ‘count++’ adds 1 well enough, but how can I add 10?

help appreciated :stare: