timestamp from javascript date

Hi everybody, here's simple extension for javascript Date object. It returns timestamp in seconds in local time. Locale is taken from users browser.

Date.prototype.getTimestamp = function () 
{
        var date = this;
        var timestamp = Math.round(date.getTime()/1000);
        timestamp = timestamp - (date.getTimezoneOffset () * 60); 
    
        return timestamp;
}

usage is simple:

var new_Date = Date();
alert(date.getTimestamp());