Leap year checker in javascript

I’m trying to put a leap year checker in my Age calculator. It worked for sometime and now it outputs "This is a leap year" every time I select a date. What am I doing wrong?? Thank you!

var year;

function ageCalculator() {
  var userinput = document.getElementById("DOB").value;
  var dob = new Date(userinput);

  if (userinput == null || userinput == '') {
    document.getElementById("message").innerHTML = "**Choose a date please!";
    return false;
  } else {
    //calculate month difference from current date in time
    var month_diff = Date.now() - dob.getTime();

    //convert the calculated difference in date format
    var age_dt = new Date(month_diff);

    //extract year from date
    var year = age_dt.getUTCFullYear();
    calYear();
    //now calculate the age of the user
    var age = Math.abs(year - 1970);

    //display the calculated age
    return document.getElementById("result").innerHTML =
      "Age is: " + age + " years. ";
  }
}

function calYear() {
  var yr = year;
  var yr = document.getElementsByName("year");
  if (yr % 400 == 0 || (yr % 100 != 0 && yr % 4 == 0)) {
    window.alert("This is not leap!");
  } else {
    window.alert("This is a leap!");
  }
}

<input type="text" id="DOB" />
<button type="button" onclick="ageCalculator()">Calculate</button>
<span id="message"></span><br/>
<span id="result"></span>