/Date(1188594000000)/ is a string and the long numbers inside the brackets are the milliseconds since the beginning of the unix epoch time. You cannot pass that(the string as it is) to Date constructor. If you want to generate a datetime object from that value, you should remove the first 6 characters (/Date() and pass the milliseconds only
var mydate='/Date(1188594000000)/';
var dateVal= parseInt(mydate.substr(6));
var dateObj= new Date(dateVal);
console.log(dateObj);
The statement mydate.substr(6) will return a string value like "1188594000000)/" and passing this to parseInt method returns the number 1188594000000 which can be safely passed to the Date constructor.