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

Question

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(         


        

Answer 1

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.

Answer 2

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...