Enter your birthdate: Calculate Age function calculateAge() { var birthdate = document.getElementById("birthdate").value; var today = new Date(); var birthdateArray = birthdate.split("-"); var birthdateObject = new Date(birthdateArray[0], birthdateArray[1]-1, birthdateArray[2]); var age = today.getFullYear() - birthdateObject.getFullYear(); var ageMonth = today.getMonth() - birthdateObject.getMonth(); if (ageMonth < 0 || (ageMonth === 0 && today.getDate() < birthdateObject.getDate())) { age--; } alert("You are " + age + " years old."); }