Why Does JavaScript\'s Date Constructor Return October When I Provide 9 for the Month?

Navigating Month Discrepancies in JavaScript's Date Constructor

When working with dates in JavaScript, it's crucial to understand the peculiarity of how months are indexed. A recent query on Mozilla Firefox Firebug raised concerns:

var myDate = new Date(2012, 9, 23, 0, 0, 0, 0);
console.log(myDate);

Output:

Date {Tue Oct 23 2012 00:00:00 GMT-0400 (Eastern Daylight Time)}

Delving into the discrepancy, the user observed an unexpected result: JavaScript returned a date in October instead of September, as intended.

Unveiling the Truth: Zero-Based Indexing

The key lies in the zero-based indexing used by JavaScript's Date constructor for months. In this indexing scheme, months start with 0 for January, and 11 represents December. Therefore, the month index passed in the constructor (9) actually corresponds to October, not September.

Clarifying with References:

The Mozilla Developer Network (MDN) provides explicit guidance:

month: Integer value representing the month, beginning with 0 for January to 11 for December.

Additionally, the ECMAScript language specification offers a technical insight:

7. Let mn be ?(ℝ(m) modulo 12).

This line elucidates that the month input undergoes a modulo operation with 12, effectively wrapping the month index within the range of 0 to 11.

Therefore, when constructing a date in JavaScript, it's essential to remember that months are zero-indexed to avoid any confusion or unexpected outcomes.

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