Why does the month argument range from 0 to 11 in JavaScript's Date constructor?

When initializing a new Date object in JavaScript using the below call, I found out that the month argument counts starting from zero.

new Date(2010, 3, 1);  // that's the 1st April 2010!

Why does the month argument start from 0? On the other hand, the day of the month argument (last one) is a number from 1 to 31. Are there good reasons for this?

It's an old (probably unfortunate, probably dying) tradition in the programming world, see the old standard (POSIX) localtime C function http://linux.die.net/man/3/localtime

The real answer to this question, is that it was copied from java.util.Date, which also had this quirk. Proof can be found on Twitter from Brendan Eich - the guy who originally implemented JavaScript (including the Date object):


first tweet


second tweet

This happened in 1995, and JDK 1.0 was in beta. It launched in 1996. In 1997, JDK 1.1 came out which deprecated the vast majority of functions on java.util.Date, moving them over to java.util.Calendar, but even that still had zero-based months. Developers fed-up with this created the Joda-Time library, which ultimately led to java.time package that's baked in to Java 8 (2014).

In short, it took 18 years for Java to get a correctly designed date/time API built-in, but JavaScript is still stuck back in the dark ages. We do indeed have excellent libraries like Moment.js, date-fns, and js-joda. But as of now, there is nothing more than Date built-in to the language. Hopefully this will change in the near future.

Everything but the day of the month is 0 based, see here for a full list including ranges :)

It's actually the 1 based days that are the oddballs here...oddly enough. Why was this was done? I don't know...but probably happened the same meeting they got plastered and decided semicolons were optional.

There are always 12 months in a year, so early C implementations might have used a static fixed-width array with indexes 0..11.

Its like this in java too.. Probably to convert int to string (0 - jan,, 1-feb), they coded this way.. because they might have an array of string (indexed from 0) of month names and these month numbers if they start from 0, it'll be lot easier to map to the month strings..

I know it's not really an answer to the original question, but I just wanted to show you my preferred solution to this problem, which I never seem to memorize as it pops up from time to time.

The small function zerofill does the trick filling the zeroes where needed, and the month is just +1 added:

function zerofill(i) {
    return (i < 10 ? '0' : '') + i;

function getDateString() {
    const date = new Date();
    const year = date.getFullYear();
    const month = zerofill(date.getMonth()+1);
    const day = zerofill(date.getDate());
    return year + '-' + month + '-' + day;

But yes, Date has a pretty unintuitive API, I was laughing when I read Brendan Eich's Twitter.

They might've considered months to be an enumeration (first index being 0) and days not since they don't have a name associated with them.

Or rather, they thought the number of the day was the actual representation of the day (the same way months are represented as numbers in a date like 12/31), as if you could make a enumeration with numbers as the variables, but actually 0-based.

So actually, for the months, perhaps they thought the proper enumeration representation would be to use the month's name, instead of numbers, and they would've done the same if days had a name representation. Imagine if we'd say January Five, January Sixth, instead of January 5, January 6, etc., then perhaps they'd have made a 0-based enumeration for days too...

Perhaps subconsciously they thought about an enumeration for months as {January, February, ...} and for days as {One, Two, Three, ...}, except for days you access the day as a number rather than the name, like 1 for One, etc., so impossible to start at 0...

It might be a flaw, but it's also very handy when you want to represent the months or day of the week as a string you can just create an array like ['jan,'feb' ...etc][new Date().getMonth()] in stead of ['','jan',feb ...etc][new Date().getMonth()] or ['jan','feb' ...etc][new Date().getMonth()-1]

days of the month are normaly not named so you won't be making arrays with names for those. In this case 1-31 is easier to handle, so you want have to subtract 1 every time...