Setting value of datetime-local from Date

I ended up subtracting getTimezoneOffset minutes to adjust the toISOString value:

var now = new Date();
now.setMinutes(now.getMinutes() - now.getTimezoneOffset());
document.getElementById('dt').value = now.toISOString().slice(0,16);

<input id="dt" type="datetime-local" />

The toISOString function is responsible of converting your local date (new Date) into GMT.

If you don't want to use GMT then slice, you need to use the pure Date constructor and all of the getX functions, where X is (days, month, year...)

In addition, you'll need to extend the Number object with a function that will help you to return 01 instead of 1 for example, to preserve the dd/mm/yyyy, hh/mm format.

Let me call this prototype function AddZero

      <input type="datetime-local" name="name" id="1234">

     <script type="text/javascript">
       Number.prototype.AddZero= function(b,c){
        var  l= (String(b|| 10).length - String(this).length)+1;
        return l> 0? new Array(l).join(c|| '0')+this : this;
     }//to add zero to less than 10,


       var d = new Date(),
       localDateTime= [(d.getMonth()+1).AddZero(),
                d.getDate().AddZero(),
                d.getFullYear()].join('/') +', ' +
               [d.getHours().AddZero(),
                d.getMinutes().AddZero()].join(':');
       var elem=document.getElementById("1234"); 
       elem.value = localDateTime;
     </script>

See this

The following function will take a date object (e.g. new Date()) and will return a string correctly formatted for <input type='datetime-local'>.

const dateForDateTimeInputValue = date => new Date(date.getTime() + new Date().getTimezoneOffset() * -60 * 1000).toISOString().slice(0, 19)