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();
    let m1=birthDate.getMonth() + 1;
    let y1=birthDate.getFullYear();

    let today=new Date();

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

    let d3,m3,y3;

    y3=y2-y1;

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

    if(d2>=d1){
        d3=d2-d1;
    }else{
        m3--;
        d3= getDaysInMonth(y1,m1)+d2-d1;
    }
    if(m3<0){
        m3=11;
        y3--;
    }
    result.innerHTML= `You are <span>${y3}</span> years, <span>${m3}</span> months and <span>${d3}</span> days old`;
}

function getDaysInMonth(year,month){
    return new Date(year,month,0).getDate();
}