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

There are always 12 months in a year, so early C implementations might have used a static fixed-width array with indexes 0,1,2..11.

Let see the example,

//START DATE CONSTRUCTOR

//IF MONTH AND DAY LESS THEN 9, IT SHOULD BE 01, 02,...,09

var zerofill = function(i) {

    return (i < 10 ? '0' : '') + i;

}

var getDateString = function (date) {

    if (date != undefined && date != null) {

        var d = zerofill(date.getDate());

        var m = zerofill(date.getMonth() + 1); ////Month from 0 to 11, so added 1 months

        var y = date.getFullYear();

        return d + "/" + m + "/" + y;

    }

}

var nowDate = new Date();

//CALL DATE STRING with DD/MM/YYYY format

getDateString(nowDate);

//END DATE CONSTRUCTOR

Example in detail link...