JS - Date issue with constructor

Question

I have an issue, and I don´t know what is wrong.

new Date(2017,3,31).getDate() returns 1 (like 1.4., not 31.3.)

new Date(2017,3,30).getDate() returns 30 (as 30.3.), which is correct.

Am I missing something?

Month starts with 0 for January, and ends with 11 for December MDN: Date — t.niese
– t.niese, Commented May 3, 2017 at 15:07
3 is April and there is no April 31st. Always read the MDN documentation. — Felix Kling
– Felix Kling, Commented May 3, 2017 at 15:07
This question title is not acceptable; please make it describe the question. — Lightness Races in Orbit
– Lightness Races in Orbit, Commented May 3, 2017 at 15:29

j08691 · Accepted Answer · 2017-05-03 15:07:39Z

4

Months in JavaScript are zero-based, so Date(2017,3,31) is actually April 31st, which doesn't exist. So you end up with May 1 instead.

answered May 3, 2017 at 15:07

j08691

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3 Comments

user5066707 Over a year ago

How so "end up" with May? I think the edit doesn't make sense

j08691 Over a year ago

@Matheus - JavaScript says hey, there is no April 31, so here's the correct day. As MDN notes: "if values are greater than their logical range (e.g. 13 is provided as the month value or 70 for the minute value), the adjacent value will be adjusted. E.g. new Date(2013, 13, 1) is equivalent to new Date(2014, 1, 1), both create a date for 2014-02-01 (note that the month is 0-based). "

Ondra Pernica Over a year ago

aaahh a thought that it is not index (zero) based.. how could I miss that? :D thank you

Mistalis · Accepted Answer · 2017-05-03 15:13:03Z

2

You probably expect 3 to be March as it is the third month of the year.

JavaScript months start at 0:

0 - January
1 - February
2 - March
...

You are trying to create the April 31st, which does not exist. Change it to:

new Date(2017, 2, 31).getDate(); // March 31st

edited May 3, 2017 at 15:13

answered May 3, 2017 at 15:10

Mistalis

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3 Comments

Mistalis Over a year ago

@OndraPernica Well, I think this constructor is ambiguous, you are not the first one to do this mistake... :)

Ondra Pernica Over a year ago

it´s so missleading. Days starts at 1, months at 0.. but ill remember that since now

mfeineis Over a year ago

There is a long tradition of people making this mistake, JavaScript copied it from Java, Java copied it from C... isn't it obvious ;-)

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