before you get too excited this was written in 2011
if I were to do this these days I would use Intl.DateTimeFormat. Here is a link to give you an idea of what type of support this had in 2011
Date.getTimezoneOffset()
The getTimezoneOffset() method returns the time difference between Greenwich Mean Time (GMT) and local time, in minutes.
For example, If your time zone is GMT+2, -120 will be returned.
Note: This method is always used in conjunction with a Date object.
var d = new Date()
var gmtHours = -d.getTimezoneOffset()/60;
document.write("The local time zone is: GMT " + gmtHours);
//output:The local time zone is: GMT 11