this turned out to be a really annoying problem. if your local is sent to US the dates validate as US date (mm/dd/yyyy) - but the available calendars input the date as (dd/mm/yyyy) so validation fails on any day over 12 (and gets the date wrong anyway if it is under 12). my solution was to use the datepickerfield from from dataobjectmanager but I had to alter line 6 of "dataobject_manager/code/date_picker_field/DatePickerField.php" from

static $dateFormat = "dd/mm/yy";
to
static $dateFormat = "mm/dd/yy";

so US dates work.

fyi - i was able to avoid modifying the core code and set the US format in the main _congif.php file with the following:

DatePickerField::set_date_format('mdy');

this seems to work when using DatePickerField on pages and dataobjects.

that works, and should be added to this page somehow: http://doc.silverstripe.org/datepickerfield