The below should work properly, and for all browsers (thanks to @MattJohnson for the tip)

Date.prototype.toIsoString = function() {
    var tzo = -this.getTimezoneOffset(),
        dif = tzo >= 0 ? '+' : '-',
        pad = function(num) {
            var norm = Math.floor(Math.abs(num));
            return (norm < 10 ? '0' : '') + norm;
        };
    return this.getFullYear() +
        '-' + pad(this.getMonth() + 1) +
        '-' + pad(this.getDate()) +
        'T' + pad(this.getHours()) +
        ':' + pad(this.getMinutes()) +
        ':' + pad(this.getSeconds()) +
        dif + pad(tzo / 60) +
        ':' + pad(tzo % 60);
}

var dt = new Date();
console.log(dt.toIsoString());