JSFiddle - React, Tailwind, and code Playground

JavaScript

var time = '17:40';
    var date = '2012-09-12';

    var d = new Date()
    var n = d.getTimezoneOffset();
    var timeZone = Math.floor( Math.abs( n/60 ) );
    var timeZoneString = (d.getTimezoneOffset() < 0 ? '-' : '+' ) + ( timeZone < 10 ? '0' + timeZone : timeZone ) + ':00';

    
    var datetime = date + 'T' + time + ':00' + timeZoneString;
    alert( datetime );