function calculate_age(dob) {   
    var diff_ms = Date.now() - dob.getTime();  
    var age_dt = new Date(diff_ms);   
    
    return Math.abs(age_dt.getUTCFullYear() - 1970);  
}  
  
console.log(calculate_age(new Date(1982, 11, 4)));  
  
console.log(calculate_age(new Date(1962, 1, 1)));