JavaScript

              
const ageDate = document.querySelector(".ageDate");
const ageCalc = document.querySelector(".ageCalc");
ageCalc.addEventListener("click", (e) => {
  if (ageDate.value != '' ) {
    let birthday = new Date(ageDate.value + 'T00:00:00.000Z');
    
    const d = new Date();
    let year = d.getFullYear();
    let month = clockFormat(d.getMonth() + 1);
    let date = clockFormat(d.getDate());
    let today = new Date(`${year}-${month}-${date}T00:00:00.000Z`);
    
    let years = today.getFullYear() - birthday.getFullYear();
    birthday.setFullYear(today.getFullYear());
    
    if (today < birthday){
        years--;
    }
    
    document.querySelector(".age-result").innerHTML = "Korea Age " + years;

  }
});

const clockFormat = (t) => {
  return String(t).padStart(2, '0');
};