differenceInYears() returns -0 when comparing the same date

```
differenceInYears('2018-02-01',` '2018-02-01')
// -0

differenceInYears(new Date(), new Date())
// -0

differenceInYears('2018-02-02', '2018-02-01')
// 0
```

Shouldn't the same year always be returned as a "positive" zero value?

Solvable by using `Math.abs(value)` if the value is 0. Perhaps a bigger discussion if _all_ values should be returned as positive numbers.