Calculating user birth information
Everything looks good. 

That said, array.indexOf wasn't supported in Internet Explorer until version 9.

If you need that support, the Mozilla Developer Network has a handy function you can include in your code to provide support for those browsers:

if (!Array.prototype.indexOf) {
    Array.prototype.indexOf = function (searchElement /*, fromIndex */ ) {
        "use strict";
        if (this == null) {
            throw new TypeError();
        }
        var t = Object(this);
        var len = t.length &gt;&gt;&gt; 0;
        if (len === 0) {
            return -1;
        }
        var n = 0;
        if (arguments.length &gt; 0) {
            n = Number(arguments[1]);
            if (n != n) { // shortcut for verifying if it's NaN
                n = 0;
            } else if (n != 0 &amp;&amp; n != Infinity &amp;&amp; n != -Infinity) {
                n = (n &gt; 0 || -1) * Math.floor(Math.abs(n));
            }
        }
        if (n &gt;= len) {
            return -1;
        }
        var k = n &gt;= 0 ? n : Math.max(len - Math.abs(n), 0);
        for (; k &lt; len; k++) {
            if (k in t &amp;&amp; t[k] === searchElement) {
                return k;
            }
        }
        return -1;
    }
}