Why is getDay() and getMonth() returning wrong values

September 16, 2022

I’m trying to turn a date which I’m getting into date + 1 day and then format it in a specific way. The first part works, but when I want to format that new date, I’m actually getting a different date than what I have initially after the increase.

var customDate = '/Date(1643587200000)/'
var number = customDate.replace(/\D/g,'');
var plusDay = new Date(parseInt(number));
console.log(plusDay) // this is the date before the increase
plusDay.setDate(plusDay.getDate() + 1);
console.log(plusDay); // this is the date I want

//this is the part where things start going wrong
var d = plusDay.getDay();
console.log(d);
var m = plusDay.getMonth();
console.log(m);
var y = plusDay.getFullYear();
console.log(y);

let fDate = new Date(y, m, d);
let ye = new Intl.DateTimeFormat('en', { year: 'numeric' }).format(fDate);
let mo = new Intl.DateTimeFormat('en', { month: 'long' }).format(fDate);
let da = new Intl.DateTimeFormat('en', { day: '2-digit' }).format(fDate);
console.log (mo + ' ' + da + ', ' + ye);

What am I doing wrong here and how do I fix it? Why is it that if I have 31. January 2022 (/Date(1643587200000)/) as customDate, the result is 2. February 2022 (+1 day), but when it’s 31. July 2021 (/Date(1627689600000)/), the resulting date is 31. July 2021 (-1 day)?

>Solution :

getDay returns the day of the week, not the day of the month. Replace it by getDate.

P.S. Also your code is not considering the timezone offset.

var customDate = '/Date(1643587200000)/'
var number = customDate.replace(/\D/g,'');
var plusDay = new Date(parseInt(number));
console.log(plusDay) // this is the date before the increase
plusDay.setDate(plusDay.getDate() + 1);
console.log(plusDay); // this is the date I want

//this is the part where things start going wrong
var d = plusDay.getDate(); // modified
console.log(d);
var m = plusDay.getMonth();
console.log(m);
var y = plusDay.getFullYear();
console.log(y);

let fDate = new Date(y, m, d);
let ye = new Intl.DateTimeFormat('en', { year: 'numeric' }).format(fDate);
let mo = new Intl.DateTimeFormat('en', { month: 'long' }).format(fDate);
let da = new Intl.DateTimeFormat('en', { day: '2-digit' }).format(fDate);
console.log (mo + ' ' + da + ', ' + ye);