What you're receiving back is a Unix epoch, which represents the number of milliseconds that have elapsed since Jan. 01 1970. Fortunately, JavaScript also uses this as its epoch, meaning you can create a new Date object using elapsed milliseconds as your input parameter:
var myDate = new Date(1363708765000);
console.log("UTC:" + myDate.toUTCString());
// outputs UTC: Tue, 19 Mar 2013 15:59:25 GMT
Of course, you'll have to parse out the integer portion of the date value you're getting back before you can use it to initialize a date. A reusable function might look like this:
function parseJsonDate( sDate ) {
var b, e, i;
b = sDate.indexOf('(');
e = sDate.indexOf(')');
i = sDate.substring(b+1,e);
if (isNaN(i)) { return null };
return new Date(parseInt(i));
}