my question about html & JS and date I want make formula calculate the age of person

Question

the user will input his/her age I think with <input type="date"/> tag and JV will calculate the age according this formula year = 2020 - (input year)
month = 12 - (input month)
day = 30 - (input day)
what I know is the all websites use some way to calculate age with the user input birthday.
I wrote all program but just I'm stuck in this code
thank you

Answer 1

try this: (This is not accurate because not all months have 30 days)
var date = document.getElementById("date");
var sub = document.getElementById("submit");
sub.addEventListener("click", calc);
function calc() {
var currentDate = new Date();
var dateArray = date.value.split("-")
console.log(currentDate)
var years = currentDate.getFullYear() - dateArray[0];
var months = currentDate.getMonth() + 1 - dateArray[1];
var days = currentDate.getDate() - dateArray[2];
if(days < 0) {
days = 30 + days;
months--;
};
if(months < 0) {
months = 12 + months;
years--;
};
alert("you are " + years + " years " + months + " months and " + days + " days old." )
}

Answer 2

thank you so much it's work yochanan sheinberger 🤗