JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="dob" name="dob"/>
<input type="button" onclick="ageCount()" value="Calc Age"/>
        <input type="text" id="ageId" name="ageId"/>

JavaScript

function ageCount() {
                var date1 = new Date();
                var dob = document.getElementById("dob").value;
                var date2 = new Date(dob);
                var pattern = /^\d{4}-\d{1,2}-\d{1,2}$/;
                //Regex to validate date format (yyyy-mm-dd)       
                if (pattern.test(dob)) {
                    var y1 = date1.getFullYear();
                    //getting current year            
                    var y2 = date2.getFullYear();
                    //getting dob year            
                    var age = y1 - y2;
                    //calculating age                       
                    document.getElementById("ageId").value = age;
                    doucment.getElementById("ageId").focus();
                    return true;
                } else {
                    alert("Invalid date format. Please Input in (yyyy-mm-dd)  format!");
                    return false;
                }
            }