What is returned is milliseconds since epoch. You could do:
var d = new Date();
d.setTime(1245398693390);
document.write(d);
On how to format the date exactly as you want, see full Date
reference at http://www.w3schools.com/jsref/jsref_obj_date.asp
You could strip the non-digits by either parsing the integer (as suggested here):
var date = new Date(parseInt(jsonDate.substr(6)));
Or applying the following regular expression (from Tominator in the comments):
var jsonDate = jqueryCall(); // returns "/Date(1245398693390)/";
var re = /-?d+/;
var m = re.exec(jsonDate);
var d = new Date(parseInt(m[0]));
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