Javascript Date toJSON() outputting incorrect date

I would use moment.js for that.

var date = moment("Fri Aug 26 2016 09:16:16 GMT+1000");
console.log(moment(date).format('YYYY-MM-DD T hh:mm:ss'));

https://jsfiddle.net/Refatrafi/ys4nu8o9/

toJSON() returns timestamps in ISO 8601 format. Z at the end of string means that used UTC. Date objects in ECMAScript are internally UTC. The specification for Date.prototype.toJSON says that it uses Date.prototype.toISOString, which states that "the timezone is always UTC".

The date isn't wrong, it's in UTC. Without timezone information, yyyy-mm-ddThh:mn:ss is meaningless unless you explicitly want to assume that it's in the AEST timezone.

If you're transmitting the date as a string to be parsed back into some sort of Date-like object later on (by your webserver, for example), there's nothing you need to do. 2016-08-25T23:20:08.242Z unambiguously refers to the same point in time no matter what you use to parse it.

If you're trying to format the date object and display it somewhere, you can extract the different parts of the Date object and build up the representation you want:

function format_date(d) {
    var pretty_date = [d.getFullYear(), d.getMonth() + 1, d.getDate()].join('-');
    var pretty_time = [d.getHours(), d.getMinutes(), d.getSeconds()].join(':');

    return pretty_date + 'T' + pretty_time;
}

As the other answers have pointed out, if you plan on working more with dates, consider using a library that makes it easier. JavaScript doesn't have a very rich API, so you'll have to write more code.