Just leverage the built-in toISOString method that brings your date to the ISO 8601 format:

let yourDate = new Date()
yourDate.toISOString().split('T')[0]

Where yourDate is your date object.

Edit: @exbuddha wrote this to handle time zone in the comments:

const offset = yourDate.getTimezoneOffset()
yourDate = new Date(yourDate.getTime() - (offset*60*1000))
return yourDate.toISOString().split('T')[0]

I hope this is what you want:

const today = new Date();
const yyyy = today.getFullYear();
let mm = today.getMonth() + 1; // Months start at 0!
let dd = today.getDate();

if (dd < 10) dd = '0' + dd;
if (mm < 10) mm = '0' + mm;

const formattedToday = dd + '/' + mm + '/' + yyyy;

document.getElementById('DATE').value = formattedToday;

How do I get the current date in JavaScript?

.getMonth() returns a zero-based number so to get the correct month you need to add 1, so calling .getMonth() in may will return 4 and not 5.

So in your code we can use currentdate.getMonth()+1 to output the correct value. In addition:

• .getDate() returns the day of the month <- this is the one you want
• .getDay() is a separate method of the Date object which will return an integer representing the current day of the week (0-6) 0 == Sunday etc

so your code should look like this:

var currentdate = new Date(); 
var datetime = "Last Sync: " + currentdate.getDate() + "/"
                + (currentdate.getMonth()+1)  + "/" 
                + currentdate.getFullYear() + " @ "  
                + currentdate.getHours() + ":"  
                + currentdate.getMinutes() + ":" 
                + currentdate.getSeconds();

JavaScript Date instances inherit from Date.prototype. You can modify the constructor's prototype object to affect properties and methods inherited by JavaScript Date instances

You can make use of the Date prototype object to create a new method which will return today's date and time. These new methods or properties will be inherited by all instances of the Date object thus making it especially useful if you need to re-use this functionality.

// For todays date;
Date.prototype.today = function () { 
    return ((this.getDate() < 10)?"0":"") + this.getDate() +"/"+(((this.getMonth()+1) < 10)?"0":"") + (this.getMonth()+1) +"/"+ this.getFullYear();
}

// For the time now
Date.prototype.timeNow = function () {
     return ((this.getHours() < 10)?"0":"") + this.getHours() +":"+ ((this.getMinutes() < 10)?"0":"") + this.getMinutes() +":"+ ((this.getSeconds() < 10)?"0":"") + this.getSeconds();
}

You can then simply retrieve the date and time by doing the following:

var newDate = new Date();
var datetime = "LastSync: " + newDate.today() + " @ " + newDate.timeNow();

Or call the method inline so it would simply be -

var datetime = "LastSync: " + new Date().today() + " @ " + new Date().timeNow();