Update DateTime.js

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date
The time-zone offset is the difference, in minutes, between UTC and local time.

I think it would be rather. 

For example: I have XML <customerOrder updated="2014-05-10T00:00:00.000+04:00"></customerOrder>, from my server in Moscow. And I have to get customerOrder.updated.toString() -> "Sat May 10 2014 02:00:00 GMT+0600" in Ekaterinburg (UTC+06:00) or in XML - <customerOrder updated="2014-05-10T02:00:00+06:00"/>