Question

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.

Answer 1

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.