How do ticks work

times.js

#!/usr/bin/env node
 // @flow
var start = Date.now();
var timer = setTimeout(() => {
    console.log('done');
    console.log(Date.now() - start);
}, 1);
var timer2 = setTimeout(() => {
    console.log(timer);
    console.log(Date.now() - start);
}, 0);
console.log(timer);