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181

`date.setMonth` causes the month to be set too high if `date` is at the end of the month

rated 0 times [ 183] [ 2] / answers: 1 / hits: 55954 / 12 Years ago, mon, february 4, 2013, 12:00:00

A Date object’s getMonth() method seems to have a bug. Assuming the Date d is 2013-01-31, I attempt to set the month on d like this:

const d = new Date(); // 2013-01-31



d.setMonth(8);

console.log(d.getMonth());

The result is 9. Why? I tested this both in Chrome and Firefox.

I found out that when it’s the 31st, 30th, or 29th of a month, setting the date to a month that has a fewer number of days causes getMonth to return the wrong value.

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jaelynncherokeeg

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answered 12 Years ago donaldcristianl · Accepted Answer

Let's break this down:

var d = new Date(); // date is now 2013-01-31

d.setMonth(1);      // date is now 2013-02-31, which is 3 days past 2013-02-28

x = d.getMonth();   // what to do, what to do, 3 days past 2013-02-28 is in March

                    // so, expect x to be March, which is 2

This is only an issue when the day value of d is greater than the maximum number of days in the month passed to setMonth(). Otherwise, it works as you'd expect.