Adding Months to a Date in JavaScript

let start = new Date();
let end = new Date(start);
end.setMonth(end.getMonth() + 1);

console.log(start, end)

I want to add months into a date and get in mm/dd/yy format

First you'll need to get your date into a Date object.

var myDate = new Date("2016-04-01 00:00:00");

Date needs the input to be formatted in the ISO format.

You can then use the following to add different numbers of months:

var myDate_6Months = new Date(myDate.setMonth(myDate.getMonth() + 6));

The line above creates a new Date object by grabbing your original date's month, and adding a desired amount of months to it.

If you were to output myDate_6Months you would get:

Sat Oct 01 2016 00:00:00

To add 11 months:

var myDate_11Months = new Date(myDate.setMonth(myDate.getMonth() + 11))

which would result in:

Wed Mar 01 2017 00:00:00

In order to get the date back in the mm/dd/yy one easy method is to use moment.js.

moment(myDate_6Months).format('MM/DD/YY')

Adding months to a Date in JavaScript

You need to get business rules for adding months. The simple solution is:

function addMonths(dateObj, num) {
  return dateObj.setMonth(dateObj.getMonth() + num);
}

However, that will change 31 July to 31 September, which will be converted to 1 October. Also, 31 January plus 1 month is 31 February which will be converted to 2 or 3 March depending on whether it's a leap year or not.

You might expect the first to be 30 September and the second to be 28 or 29 February (depending on whether it's a leap year or not).

So if you want "end of months" be observed, you need to do something like:

function addMonths(dateObj, num) {

    var currentMonth = dateObj.getMonth() + dateObj.getFullYear() * 12;
    dateObj.setMonth(dateObj.getMonth() + num);
    var diff = dateObj.getMonth() + dateObj.getFullYear() * 12 - currentMonth;

    // If don't get the right number, set date to 
    // last day of previous month
    if (diff != num) {
        dateObj.setDate(0);
    } 
    return dateObj;
} 

But check with whoever is responsible for the business rules that that is what they want.

Edit

The above works well, but in response to McShaman's comment here is a version with a simpler check for the month roll–over:

function addMonths(date, months) {

  var d = date.getDate();

  date.setMonth(date.getMonth() + +months);

  if (date.getDate() != d) {

    date.setDate(0);

  }

  return date;

}


// Add 12 months to 29 Feb 2016 -> 28 Feb 2017

console.log(addMonths(new Date(2016,1,29),12).toString());


// Subtract 1 month from 1 Jan 2017 -> 1 Dec 2016

console.log(addMonths(new Date(2017,0,1),-1).toString());


// Subtract 2 months from 31 Jan 2017 -> 30 Nov 2016

console.log(addMonths(new Date(2017,0,31),-2).toString());


// Add 2 months to 31 Dec 2016 -> 28 Feb 2017

console.log(addMonths(new Date(2016,11,31),2).toString());

var d = new Date();
d.setMonth(d.getMonth() - 3);

var d = new Date("January 14, 2012");

console.log(d.toLocaleDateString());

d.setMonth(d.getMonth() - 3);

console.log(d.toLocaleDateString());