Why getMonth returns different values on same date with different date formats

Question

The Date.getMonth() returns different values for same date in different formats.

The first statement is in the UTC format.

The second one uses YYYY,M,DD format.

I didn't expect it to return a different value because it's the same date. What is going on?

console.log(new Date('July 20, 69 00:20:18').getMonth()); // returns 6
console.log(new Date(1969, 7, 20).getMonth()); // returns 7

Does this answer your question? [Why does Date.parse give incorrect results?](https://stackoverflow.com/questions/2587345/why-does-date-parse-give-incorrect-results) (`new Date(string)` calls `Date.parse` internally) — Heretic Monkey, Feb 13 '20 at 17:40
Also, how are July 20 1969 and August 20 1980 at all the same date? — Heretic Monkey, Feb 13 '20 at 17:42
Yes it answers my question. fixed the year, but how come 7th month is august? — anoop chandran, Feb 13 '20 at 17:44
Months are counted from zero (January = 0, February = 1, etc.) — Heretic Monkey, Feb 13 '20 at 17:46
In the second example it considers the month “7” to be August? — evolutionxbox, Feb 13 '20 at 17:47

score 1 · Answer 1 · answered Feb 13 '20 at 17:45

For your first example - read MDN docs for Date.getMonth(). It returns a zero-indexed numerical month representation where January is represented by 0, and December by 11. Thus, July is represented (correctly) by 6.

The second one doesn't use a format per se, it's passing the values as parameters to the Date constructor. The MDN docs for Date() state that the parameter you're passing with a value 7 is also a monthIndex. Thus, to correctly notate July, pass 6 instead.

score 1 · Answer 2 · answered Feb 13 '20 at 17:45

1

The getMonth method returns a zero based value. Meaning 0 indicates the first month. Creating a new Date object like in your second example follows the same principle.

See these examples.

answered Feb 13 '20 at 17:45

Kevin

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score 0 · Answer 3 · answered Feb 13 '20 at 17:48

The months in a Date object are zero indexed. That means they range from 0 to 11.

new Date('July 20, 69 00:20:18') creates a Date with the month of July which has the index 6.

new Date(1980, 7, 20) creates a date with the month with the 7th index, which is August.

Here you can find more details: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/Date

score 0 · Answer 4 · answered Feb 13 '20 at 17:59

That diference happens because when using integer parameters to new Date(1980, 6, 20) the month parameter starts in 0, not 1. example:
0 - january
1 - february
...
5 - july
6 - august
...
11 - december

So your result is

var dateString = new Date('July 20, 69 00:20:18') // "1969-07-20T03:20:18.000Z"
var dateInt = new Date(1980, 7, 20) // "1980-08-20T03:00:00.000Z"

console.log(dateString.getMonth()); // 6
console.log(dateInt.getMonth()); // 7

Why getMonth returns different values on same date with different date formats

4 Answers4