JSFiddle - React, Tailwind, and code Playground

JavaScript

var birthday = new Date('Dec,15,1992');
var today = new Date();

var calculatedDate = calculate(normalizeDate(birthday), normalizeDate(today));

document.write(
		'years: ' + calculatedDate.year + '<br>' +
    'months: ' + calculatedDate.month + '<br>' +
    'days: ' + calculatedDate.day);

function calculate(firstDate, currentDate) {
     return {
         year: currentDate.year - firstDate.year,
         month: currentDate.month - firstDate.month,
         day: currentDate.day - firstDate.day,
     }
}

function normalizeDate(date) {
  return {
    year: date.getFullYear(),
    month: date.getMonth() + 1,
    day: date.getDate()
  };
}