How to Add Months to a Date in JavaScript

How to add months to a date in JavaScript?

Corrected as of 25.06.2019:

var newDate = new Date(date.setMonth(date.getMonth()+8));

Old
From here:

var jan312009 = new Date(2009, 0, 31);
var eightMonthsFromJan312009  = jan312009.setMonth(jan312009.getMonth()+8);

JavaScript function to add X months to a date

The following function adds months to a date in JavaScript (source). It takes into account year roll-overs and varying month lengths:

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 2017console.log(addMonths(new Date(2016,1,29),12).toString());
// Subtract 1 month from 1 Jan 2017 -> 1 Dec 2016console.log(addMonths(new Date(2017,0,1),-1).toString());
// Subtract 2 months from 31 Jan 2017 -> 30 Nov 2016console.log(addMonths(new Date(2017,0,31),-2).toString());
// Add 2 months to 31 Dec 2016 -> 28 Feb 2017console.log(addMonths(new Date(2016,11,31),2).toString());

Adding months to date in js

By calling start.setMonth you end up updating the month on both dates. (This is noted in one of the comments on the answer you followed, but not in the answer itself.)

Separate the statements to only affect the date you want changed:

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

console.log(start, end)

Add months to a date is not working in JavaScript

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())

Add months in local language to date function

var $time = "Fri May 04 20:33:17 +0000 2018";

function relative_time(time_value) {
    var date = new Date(time_value);
    var months= ['Enero', 'Febrero', 'Marzo', 'Abril', 'Mayo', 'Junio', 'Julio', 
    'Agosto', 'Septiembre', 'Octubre', 'Noviembre', 'Diciembre'];
    return date.getDate()+ ' '+ months[date.getMonth()];
   }

relative_time($time);

Does this answer your question?

Add N months to YYYY-MM date in JavaScript

The main challenge is due to the fact that we count months between 1 and 12, while a modulo of 12 have a valid range of 0 to 11.

We can solve this by first subtracting 1 from the month count so that its range can fit within the modulo's range, and add the 1 back after the modulo calculation.

function addMonths(time, monthsToAdd) {
  const offsetCycles = Math.ceil(Math.abs(monthsToAdd) / 12);
  const zeroBasedMonth = time.month + monthsToAdd - 1 + offsetCycles * 12;
  const additionalYears = Math.floor(zeroBasedMonth / 12) - offsetCycles;

  return {
    year: time.year + additionalYears,
    month: (zeroBasedMonth % 12) + 1,
  };
}

console.log([
  addMonths({ year: 2001, month: 1 }, 2),
  addMonths({ year: 2001, month: 11 }, 2),
  addMonths({ year: 2001, month: 11 }, 1),
  addMonths({ year: 2001, month: 11 }, 12),
  addMonths({ year: 2001, month: 11 }, 14),
  addMonths({ year: 2001, month: 1 }, -1),
  addMonths({ year: 2001, month: 1 }, -13),
]);

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