これを達成できるカスタム関数は、Asi dintの答えです。他の人に役立つことを願っています
//alert(dateText);
var theDate = new Date(Date.parse($(this).datepicker('getDate')));
var maxdate = $.datepicker.formatDate('d-m-yy', theDate);
var theMinDate = new Date(Date.parse($("#datepicker_1").datepicker('getDate')));
var mindate = $.datepicker.formatDate('d-m-yy', theMinDate);
alert(mindate + "\n" + maxdate);
var stamp1 = str2date(mindate).getTime(); //First date. getTime() converts it to an integer
var stamp2 = str2date(maxdate).getTime(); //Second date
//alert(stamp1);
//alert(stamp2);
for(var i=0; i<unavailableDates.length; i++){
var curStamp = str2date(unavailableDates[i]).getTime();
if(curStamp >= stamp1 && curStamp <= stamp2) //Check if it falls in range
alert(unavailableDates[i] + " falls in range");
}