Adding 3 days to a specific date in Javascript

Question

I'm trying to add 3 days to a random date but instead it seems as though I'm adding a month.

var d = new Date(2021, 9, 14);
var currentTime = d.getTime();
var daysToAdd = 3;
var secondsInADay = 86400;
var d = new Date(currentTime + daysToAdd * secondsInADay);
var year = d.getFullYear();
var month = ("0" + (d.getMonth() + 1)).slice(-2);
var day = ("0" + d.getDate()).slice(-2);
console.log('result is:' + year + '-' + month + '-' + day);

Why not just set the day to the original day plus 3? `d.setDay(d.getDay() + 3)`? — Pointy, Nov 13 '21 at 19:27

score 0 · Answer 1 · answered Nov 13 '21 at 19:40

Try this :

var d = new Date(2021, 9, 14) ;
    var daysToAdd = 3;
    
    Date.prototype.addDays = function(days) {
        var date = new Date(this.valueOf());
        date.setDate(date.getDate() + days);
        return date;
    }
    console.log(date.addDays(daysToAdd));

score 0 · Answer 2 · answered Nov 13 '21 at 19:46

You're multiplying by the number of seconds in a day, but you need to multiply by the number of milliseconds in a day, as shown below:

var d = new Date(2021, 9, 14);
var currentTime = d.getTime();
var daysToAdd = 3;
var milisecondsInADay = 86400000;
var d = new Date(currentTime + daysToAdd * milisecondsInADay);
var year = d.getFullYear();
var month = ("0" + (d.getMonth() + 1)).slice(-2);
var day = ("0" + d.getDate()).slice(-2);
console.log('result is:' + year + '-' + month + '-' + day);

Adding 3 days to a specific date in Javascript

2 Answers2