JS timezone conversion

In Javascript, I have a timestamp which I'm handling like so:

var origUnixTimestamp = (date * 1000);

Along with this timestamp I have a UTC offset (-5, although this is variable). I'm looking to convert origUnixTimestamp to the users UTC offset, using Date's getTimezoneOffset() method.

I'm just wondering how I take into account the original timestamps UTC offset (-5, for example) and convert it to the users current UTC offset. I imagine this is fairly simple, but it's warping my brain at the moment.

Javascript doing it for you. All the dates stored in the Date object are already converted to the correct timezone (just pass your epoch to the constructor). The same Date object has ability to work with epoch date and UTC.

var some_date = new Date(epoch);
var time = some_date.getDay(); // will be different in different zones
some_date.setDay(22); // to set day
var origUnixTimestamp = some_date.getTime(); //returns you epoch

How about this: http://github.com/mde/timezone-js

This link has instructions for converting from local time:

// create Date object for current location
d = new Date();

// convert to msec
// add local time zone offset
// get UTC time in msec
utc = d.getTime() + (d.getTimezoneOffset() * 60000);