**this is an approximation!** if the diff is 15 days on a 31 day month the result will be different than 15 days on a 28 day month.

```js
15 / 31 / 12 = 0.04032258064516129
15 / 28 / 12 = 0.044642857142857144
```

the margin of error should be between `0.0002` and `0.007` which for me seems reasonable given the amount of work required to get the accurate result when the dates spans through multiple years and some of them are leap years.