Adding months to a Date in JavaScript [duplicate]

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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());

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