add or subtract timezone difference to javascript Date

You can use Date.getTimezoneOffset which returns the local offset from GMT in minutes. Note that it returns the value with the opposite sign you might expect. So GMT-5 is 300 and GMT+1 is -60.

var date = "September 21, 2011 00:00:00";
var targetTime = new Date(date);
var timeZoneFromDB = -7.00; //time zone value from database
//get the timezone offset from local time in minutes
var tzDifference = timeZoneFromDB * 60 + targetTime.getTimezoneOffset();
//convert the offset to milliseconds, add to targetTime, and make a new Date
var offsetTime = new Date(targetTime.getTime() + tzDifference * 60 * 1000);

If you need to compensate the timezone I would recommend the following snippet:

var dt = new Date('2018-07-05')
dt.setMinutes(dt.getMinutes() + dt.getTimezoneOffset())
console.log(dt)

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/getTimezoneOffset

The getTimezoneOffset() method returns the difference, in minutes, between a date as evaluated in the UTC time zone, and the same date as evaluated in the local time zone.

So all you need is to compensate, IN MINUTES

Simple function that works for me:

adjustForTimezone(date:Date):Date{
    var timeOffsetInMS:number = date.getTimezoneOffset() * 60000;
    date.setTime(date.getTime() + timeOffsetInMS);
    return date
}

This example shows how to use the local datetime but format it as ISO:

const d = new Date();

let dtOffset = new Date(d.setMinutes(d.getMinutes() - d.getTimezoneOffset()));
// Date in EST and ISO format: "2021-11-30T15:33:32.222Z"
console.log(dtOffset.toISOString());