Add months to a date is not working in JavaScript

Question

I just want to add 45 months to the given date in JavaScript. I have tried this:

var startDateFormat = new Date(2018, 11, 24); // 11 is for January starts from 0
var addDate = startDateFormat.setMonth(startDateFormat.getMonth() + 45); //want to add 45 months
console.log(new Date(addDate).getFullYear() + "-" + new Date(addDate).getMonth() + 1 + "-" + new Date(addDate).getUTCDate())

But the result is 2019-101-23. Can anybody help me why this is happening?

Try this [link](https://stackoverflow.com/questions/5645058/how-to-add-months-to-a-date-in-javascript) — Nareen Babu, Dec 24 '18 at 10:14
The comment on the first line is wrong. January is 0, December is 11. Why call a variable `startDateFormat` when it is not a format, but a date. — trincot, Dec 24 '18 at 10:14

score 3 · Answer 1 · answered Dec 24 '18 at 10:14

There are a few problems there:

setMonth modifies the state of the instance you call it on, you don't usually use its return value.
The + 1 you're doing after getMonth is adding a "1" to the string. If you want it numerically, group it with the getMonth.
Don't combine UTC methods and non-UTC methods, use UTC in all of the calls or none of them.
You don't have to create new dates all over the place, you can just use the first one.
Not quite sure what your comment saying "11 is for January starts from 0" is meant to mean. 11 is December, not January.

So:

var dt = new Date(2018, 11, 24); // 11 is for January starts from 0
dt.setMonth(dt.getMonth() + 45); //want to add 45 months
console.log(dt.getFullYear() + "-" + (dt.getMonth() + 1) + "-" + dt.getDate());
// Note parens ----------------------^-----------------^

score 1 · Accepted Answer · answered Dec 24 '18 at 10:15

You should place new Date(addDate).getMonth() + 1 in parentheses. You are building a string and since you don't provide explicit precedence, first new Date(addDate).getMonth() gets added, then 1 gets concatenated to the string.

Try this:

var startDateFormat = new Date(2018, 11, 24);

var addDate = startDateFormat.setMonth(startDateFormat.getMonth() + 45);

console.log(new Date(addDate).getFullYear() + "-" + (new Date(addDate).getMonth() + 1) + "-" + new Date(addDate).getUTCDate())

Or template string:

var startDateFormat = new Date(2018, 11, 24);

var addDate = startDateFormat.setMonth(startDateFormat.getMonth() + 45);

console.log(`${new Date(addDate).getFullYear()}-${new Date(addDate).getMonth() + 1}-${new Date(addDate).getUTCDate()}`);

There's no point to all those `new Date`s, and in fact, you can't reliably use `new Date(dateInstance)` to clone a date cross-browser. — T.J. Crowder, Dec 24 '18 at 10:16
@T.J.Crowder True, I was simply answering OPs question about why he sees this output. — Alex G, Dec 24 '18 at 10:18

score 0 · Answer 3 · answered Dec 24 '18 at 10:54

45 months, means 3 years + 9 months. so use javascript divider and modulus operators something like this..

        var startDateFormat = new Date(2018, 11, 24); // 11 is for January starts from 0

    var year = startDateFormat.setYear(startDateFormat.getFullYear() + (45 / 12));

    if ((startDateFormat.getMonth() + 1 + (45 % 12)) > 12)
    {
        year = new Date(year).setYear(new Date(year).getFullYear() + 1);


        var month = startDateFormat.setMonth(startDateFormat.getMonth() + (45 % 12) - 12 + 1);
    }
    else
    {
        var month = startDateFormat.setMonth(startDateFormat.getMonth() + (45 % 12) + 1);
    }


    console.log(new Date(year).getFullYear() + "-" + new Date(month).getMonth() + "-" + new Date(month).getDate())

Thanks

There's no need for that at all. JavaScript's `Date` object correctly handles wrapping the units. If you add three months to December 2018, you get March 2019 *per spec*. — T.J. Crowder, Dec 24 '18 at 11:33

score -2 · Answer 4 · answered Dec 24 '18 at 10:24

Looking at your code, it seems like you need to play with dates a lot in your project. In that case, I suggest you to try momentjs library which makes playing with dates really easy.

e.g

moment([2010, 0, 31]);                  // January 31
moment([2010, 0, 31]).add(1, 'months'); // February 28

similarly, it had tons easy to use features. for more refer, http://momentjs.com/ and for documentation on the same visit http://momentjs.com/docs/

Add months to a date is not working in JavaScript

4 Answers4