var date1 = new Date('August 19, 1975 23:15:30 GMT+07:00');
var date2 = new Date('August 19, 1975 23:15:30 GMT-02:00');

console.log(date1.getTimezoneOffset());
// expected output: your local timezone offset in minutes
// (eg -120). NOT the timezone offset of the date object.

console.log(date1.getTimezoneOffset() === date2.getTimezoneOffset());
// expected output: true