Jquery Datepicker defaultDate ONLY set the default date that you chose on the calendar that pops up when you click on your field. If you want the default date to APPEAR on your input before the user clicks on the field you should give a val() to your field. Something like this:
$("#searchDateFrom").datepicker({ defaultDate: "-1y -1m -6d" });
$("#searchDateFrom").val((date.getMonth()) + '/' + (date.getDate() - 6) + '/' + (date.getFullYear() - 1));