Occasionally when you group dates in pivot tables, the sort order seems to go out of kilter and seems to sort based on the letters (August before June etc) instead of the date.
This seems to be linked to your regional setting. If you compare what your regional settings treat as a date to how it is formatted in the spreadsheet you may see a difference. If you fix this so that the format is the same, you will see that grouped dates will sort correctly.
Another common problem is where the pivot tables refuse to group by dates (or numbers). You can see what the problem is by viewing the blog post on ‘Pivot table cannot group on that selection‘.