I think the function toLocaleDateString use the default local data on the device.

try this code to check the output:

// America/Los_Angeles for the US
// US English uses month-day-year order
console.log(date.toLocaleDateString('en-US'));
// → "12/19/2012"

// British English uses day-month-year order
console.log(date.toLocaleDateString('en-GB'));
// → "20/12/2012"

// Korean uses year-month-day order
console.log(date.toLocaleDateString('ko-KR'));
// → "2012. 12. 20."

// Arabic in most Arabic speaking countries uses real Arabic digits
console.log(date.toLocaleDateString('ar-EG'));
// → "٢٠‏/١٢‏/٢٠١٢"

// chinese
console.log(date.toLocaleDateString('zh-Hans-CN'));
// → "2012/12/20"

For any one looking for a really simple ES6 solution to copy, paste and adopt:

const dateToString = d => `${d.getFullYear()}-${('00' + (d.getMonth() + 1)).slice(-2)}-${('00' + d.getDate()).slice(-2)}` 

// how to use:
const myDate = new Date(Date.parse('04 Dec 1995 00:12:00 GMT'))
console.log(dateToString(myDate)) // 1995-12-04

Hi please try th following function:

function getTime(){
    var date = new Date();
    console.log(GetFormattedDate(date));
}



function GetFormattedDate(date) {
    var month = ("0" + (date.getMonth() + 1)).slice(-2);
    var day  = ("0" + (date.getDate())).slice(-2);
    var year = date.getFullYear();
    var hour =  ("0" + (date.getHours())).slice(-2);
    var min =  ("0" + (date.getMinutes())).slice(-2);
    var seg = ("0" + (date.getSeconds())).slice(-2);
    return year + "-" + month + "-" + day + " " + hour + ":" +  min + ":" + seg;
}

in your case

function getTime(){
    var date = new Date();

    var currentdate = new Date();
    var prevdate = new Date();
    var firstdate = new Date();

    prevdate.setTime(currentdate.getTime() - (30 * 60 * 1000));
    firstdate.setTime(currentdate.getTime() + (30 * 60 * 1000));


    console.log(GetFormattedDate(prevdate));
    console.log(GetFormattedDate(currentdate));
    console.log(GetFormattedDate(firstdate));
}



function GetFormattedDate(date) {
    var month = ("0" + (date.getMonth() + 1)).slice(-2);
    var day  = ("0" + (date.getDate())).slice(-2);
    var year = date.getFullYear();
    var hour =  ("0" + (date.getHours())).slice(-2);
    var min =  ("0" + (date.getMinutes())).slice(-2);
    var seg = ("0" + (date.getSeconds())).slice(-2);
    return year + "-" + month + "-" + day + " " + hour + ":" +  min + ":" + seg;
}

regards

Per my test and search, it seems the toLocaleDateString() cannot convert date format. We can do it manually with an expression like this:

=getYear(@currentField)+'-'+(getMonth(@currentField)+1)+'-'+getDate(@currentField)

Just as OP mentioned, the getMonth returns a 0-based value, we need to add 1 for it.

getMonth: returns the month in the specified date according to local time, as a zero-based value (where zero indicates the first month of the year). - Only available in SharePoint Online

I think you would have to manually parse it into that format, which actually isn't too bad. What Date.toLocaleString() returns is a format of:

MM/DD/YYYY, HH:MM:SS

Here's my code snippet to help you out:

// Parse our locale string to [date, time]
var date = new Date().toLocaleString('en-US',{hour12:false}).split(" ");

// Now we can access our time at date[1], and monthdayyear @ date[0]
var time = date[1];
var mdy = date[0];

// We then parse  the mdy into parts
mdy = mdy.split('/');
var month = parseInt(mdy[0]);
var day = parseInt(mdy[1]);
var year = parseInt(mdy[2]);

// Putting it all together
var formattedDate = year + '-' + month + '-' + day + ' ' + time;

You can use the options argument in .toLocaleString to format your date as "MM/DD/YYYY"

var currentDate = new Date(Date.now() + (8 * 86400000))
var newDateOptions = {
        year: "numeric",
        month: "2-digit",
        day: "2-digit"
}
var newDate = currentDate.toLocaleString("en-US", newDateOptions );

console.log(newDate)

A detailed post on how to use the arguments in .toLocaleString - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/toLocaleString

You can fully format the string as mentioned in other posts. But I think your better off using the locale functions in the date object?

var d = new Date("2017-03-16T17:46:53.677"); 
console.log( d.toLocaleString() ); 

edit :

ISO 8601 ( the format you are constructing with ) states the time zone is appended at the end with a [{+|-}hh][:mm] at the end of the string.

so you could do this :

var tzOffset = "+07:00" 
var d = new Date("2017-03-16T17:46:53.677"+ tzOffset);
console.log(d.toLocaleString());
var d = new Date("2017-03-16T17:46:53.677"); //  assumes local time. 
console.log(d.toLocaleString());
var d = new Date("2017-03-16T17:46:53.677Z"); // UTC time
console.log(d.toLocaleString());

edit :

Just so you know the locale function displays the date and time in the manner of the users language and location. European date is dd/mm/yyyy and US is mm/dd/yyyy.

var d = new Date("2017-03-16T17:46:53.677");
console.log(d.toLocaleString("en-US"));
console.log(d.toLocaleString("en-GB"));