MaxDate appears to be exclusive, not inclusive, in Date Pickers, and I think that's confusing and it's also not documented. If you create a DatePicker and then set MaxDate to 9/1/2015, you cannot enter the date 9/1/2015 in the date picker. It will say this:
"The Start Date is too late. It must be between 1/1/1900 12:00:00AM and 9/1/2015 12:00:00 AM."
First, it's sort of silly to have times appear in the error message for a control that doesn't all you to select time. Second, you would clearly expect to be able to enter 9/1 in a date picker with a max date of 9/1.