Not change date

I faced with starnge behavior, startYear not changed (in moment js this code works);
```
let startYear = dayjs().subtract(15, 'years');
let endYear = dayjs();
let steps = 0;
while (endYear.diff(startYear, 'years') > 0) {
startYear.add(1, 'year');
console.log(startYear.year(), endYear.year())
steps++;
if(steps === 10) break;
}

This show:

```

**Information**
- Day.js v1.10.7
- OS: Windows
- Browser Last Chrome