JSFiddle - React, Tailwind, and code Playground

HTML

<button id="calculateDays">calculate days</button>

JavaScript

function getDaysInMonth1(m, y) {
    return m === 2 ? y & 3 || !(y % 25) && y & 15 ? 28 : 29 : 30 + (m + (m >> 3) & 1);
}

function getDaysInMonth2(m, y) {
    return new Date(y, m + 1, 0).getDate();
}

$("#calculateDays").click(function () {
    console.clear();
    for (var y = 1; y < 4000; y++) {
        for (var m = 1; m <= 12; m++) {
            var d1 = getDaysInMonth1(m, y);
            var d2 = getDaysInMonth2(m - 1, y);
            if (d1 != d2) {
                console.log("y: " + y + "m: " + m + "d1: " + d1 + " d2:" + d2);
            }
        }
    }
    console.log("done");
});