I've noticed that in JavaScript, when creating a Date, months are zero based, and days aren't.

For example:

var foo = new Date(2012, 1, 1)

produces February 1st 2012

Why is this?

  • 7
    months = ['Jan', 'Feb',..., 'Nov', 'Dec']; month = months[date.getMonth()];
    – zzzzBov
    Dec 13, 2012 at 18:15

3 Answers 3


Most likely the idea is, that the months are thought of as an index into an array of month names, while days are simply "counted".

  • 24
    Where this makes sense from an answer stand point, it's bat crap crazy to think this logic has been around for how long now and i've never questioned it before.
    – rlemon
    Dec 13, 2012 at 15:12
  • 2
    @rlemon The fact that you've never questioned that logic before only proves that it makes sense :) Dec 13, 2012 at 15:18
  • 17
    FWIW, I never thought it made much sense. Processing dates in JavaScript was always something that made me shudder, because I'd usually forget things like this until I started debugging strange results. Dec 13, 2012 at 15:20
  • 2
    FWIW, I am not a front end developer and don't often have to deal with dates in Javascript aside from the occasional new Date().getTime()
    – rlemon
    Dec 13, 2012 at 15:28
  • 22
    When it comes to Javascript, "this is why they did it" and "this makes a good sense" are two things you rarely hear together.
    – Tridus
    Dec 13, 2012 at 17:34

It may also be that Javascript dates were meant to mimic java dates. Java has similarly used a zero based month since the beginning.

JDK API v1.0.2 : Date.getMonth()

  • 7
    And the java spec mirrors the C tm structure. See also stackoverflow.com/questions/344380/…
    – user40980
    Dec 13, 2012 at 15:52
  • 36
    That's not to say that the Java Date API is a terribly good example of good design ;-) Dec 13, 2012 at 15:52

Adding to Danny Kirchmeier's answer above, as this article claims, here's the reason as explained by Brendan Eich, the creator of Javascript:

Brendan Eich on Twitter explaining why month is zero-based, and day is 1-based in Javascript

For reasons I'm not aware of, that Tweet seems to no longer exist, at least not for me (not a Twitter user).

  • Nice find! Do you think you could add the content of the tweet to your message in case the image is deleted? Nov 30, 2021 at 15:38
  • I screenshotted the image and uploaded it on SE, so it shouldn't ever get deleted.
    – connexo
    Nov 30, 2021 at 15:47
  • @connexo Famous last words! I hope you're right. :) Nov 30, 2021 at 15:59
  • This legacy originates with Taligent. Sun licensed their Calendar implementation from IBM. Fun fact: it supports 13 months so 12 is a valid month called 'Undecimber'.
    – JimmyJames
    Nov 30, 2021 at 16:28

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