JavaScript ES6 setInterval in a static class method - ecmascript-6

Question

I want to use ES6 class definition in titanium-alloy mvc. I created a class named Clock that should work like a stopwatch similar to that one. If I try to translate this Clock class into ES6 then I'm not able to display the counter. The result string in the view/index.xml is NaN:NaN:NaN. I think it is about passing $.clockLbl. I read out this article about "Using ES6+ in a Titanium App" but was not able to apply. Could someone help please.
class Clock {
/*
constructor(){
this.totalSeconds = 0;
this.isRunning = 0;
this.intervall = null;
}
*/
static start(clockLbl) {
var self = this;
this.isRunning = 1;
this.interval = setInterval(function() {
self.totalSeconds += 1;
var hours = Math.floor(self.totalSeconds / 3600);
var min = Math.floor(self.totalSeconds / 60 % 60);
var sec = parseInt(self.totalSeconds % 60);
clockLbl.text = '' + hours + ':' + min + ':' + sec;
}, 1000);
}
static pause() {
this.isRunning = 0;
clearInterval(this.interval);
delete this.interval;
}
}
Clock.totalSeconds = 0;
Clock.isRunning = 0;
Clock.intervall = null;
module.exports = Clock;
My controller/index.js:
var Clock = require('stopwatch');
Clock.start($.clockLbl);