Fix bug calculating days in duration

This bug was manifest under the following conditions:

* Calculating duration
* Duration involved days and months
* Months of the start and end dates were different lengths (30 vs 31 days)
* The end day was smaller than the start day

It used the wrong month's days when calculating the overflow for the month. As you can see below I just needed to calculate the days after the overflow of the month was performed instead of before.