Edit in JSFiddle

<div>
Today: <span id="ex1-t"></span><br />
Next Day: <span id="ex1-n"></span>
</div>
<div class="line"></div>
<div>
Today: <span id="ex2-t"></span><br />
Next Day: <span id="ex2-n"></span>
</div>
<div class="line"></div>
<div>
Today: <span id="ex3-t"></span><br />
Next Day: <span id="ex3-n"></span>
</div>
function addDay(date, day) {
		return new Date(date.getTime() + 86400000 * day);
}

function setDate(date, dateString) {
    date.setFullYear(dateString.split("-")[0]);
    date.setMonth(dateString.split("-")[1] - 1);
    date.setDate(dateString.split("-")[2]);
    return date;
}

var today = new Date();
document.getElementById('ex1-t').innerHTML = today;
document.getElementById('ex1-n').innerHTML = addDay(today, 1);

var dateString = "2018-11-11";
var myDate = new Date();
setDate(myDate, dateString);
document.getElementById('ex2-t').innerHTML = myDate;
document.getElementById('ex2-n').innerHTML = addDay(myDate, 1);

var JANLastDay = 1517328000000;
var myDate2 = new Date(JANLastDay);
document.getElementById('ex3-t').innerHTML = myDate2;
setDate(myDate2, dateString);
document.getElementById('ex3-n').innerHTML = myDate2;