SOURCE

let a = new Date('2023-09-28 17:07')

  function addDay(dayNumber, date) {
 
        date = date ? date : new Date();
 
        var ms = dayNumber * (1000 * 60 * 60 * 24)
 
        var newDate = new Date(date.getTime() + ms);
 
        return newDate;
 
    }

    b = addDay(-8,a)
    console.log(b.toString())