For the 31 day issue, it's working properly in the calendar and in the tagging date of the posts.
The strange thing happens in the Archive index page. I chose to have weekly archives, and the list's links gives the starting and eding date of each week.
If you see the first two links of the list you have: link 1 for this week: from 28 feb. to 3 march (correct) link 2 for last week: from 25 feb. to 31 february (wrong).
Also note the funny "overlapping" of the two weeks