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";
static $dateFormat = "mm/dd/yy";
so US dates work.