Impriving time_length for 29 Feb (when dealing with date time)

With previous version, we had:

```r
> naiss <- ymd_hms("1992-02-29 12:00:00")
> evt <- ymd_hms("2014-03-01 01:00:00")
> time_length(interval(naiss, evt), "years")
[1] 21.99874
```

We should consider that the anniversary occurs between 28 Feb at 23:59:59 and 1st March at 00:00:00. Therefore, `%m++%` should rollback at 00:00:00. In that case:

```r
> time_length(interval(naiss, evt), "years")
[1] 22.00011
```