script.js

let userInput = document.getElementById("date");
userInput.max = new Date().toISOString().split("T")[0];
let result = document.getElementById("result");

function calculateAge() {
    let birthDate = new Date(userInput.value);
    let d1 = birthDate.getDate();// store date
    let m1 = birthDate.getMonth() + 1;//store month
    let y1 = birthDate.getFullYear();//store year

    let today = new Date();

    let d2 = today.getDate();
    let m2 = today.getMonth() + 1;
    let y2 = today.getFullYear();

    let d3, m3, y3;

    y3 = y2 - y1;// y3=year difference, y2= current date, y1= DoY

    if (m2 >= m1) {
        m3 = m2 - m1;
    } else {
        y3--;
        m3 = 12 + m2 - m2;
    }

    if (d2 >= d1) {
        d3 = d2 - d1;
    } else {
        m3--;
        d3 = getDaysInMonth(y1, m1) + d2 - d1;
    }
    if (m3 < 0) {
        m3 = 11;
        y3--;

    }

     result.innerHTML = `You are ${y3} years, ${m3} months and ${d3} days old`;
}
function getDaysInMonth(year, month) {
    return new Date(year, month, 0).getDate();
}