What about split the input string in 2 values ?
Date foo2 = new Date(Long.parseLong("-588657600000") + Long.parseLong("-0400"));
btw, that date is : Mon May 07 16:59:59 BRT 1951 hehehe
EDIT :
this dont check the input values, and assume allways will have the minus
import java.util.Date;
public class MiMiMi {
    public static void main(String[] args) {
        String input = "/Date(-588657600000-0400)/";
        input = input.replace("/Date(", "");
        input = input.replace(")/", "");
        String[] pair = input.split("-");
        System.out.println(pair[1]);
        System.out.println(pair[2]);
        Date foo = new Date(Long.parseLong("-" + pair[1])
                + Long.parseLong("-" + pair[2]));
        System.out.println(foo);
    }
}
2
solved NumberFormatException: Invalid long: “588657600000-0400” [closed]