Tente você mesmo - JavaScript Reference month_defaultvalue3

<!DOCTYPE html>
<html>
<body>

<p>Change the month/year of the month field, and then click the button below.</p>

Month and year: <input type="month" id="myMonth" value="1997-11">

<p>Note that the default value is not affected when you change the value of the month field.</p>

<button type="button" onclick="myFunction()">Try it</button>
    
<p><strong>Note:</strong> input elements with type="month" are not supported in Internet Explorer or Firefox.</p>

<p id="demo"></p>

<script>
function myFunction() {
    var x = document.getElementById("myMonth");
    var defaultVal = x.defaultValue;
    var currentVal = x.value;
    
    if (defaultVal == currentVal) {
        document.getElementById("demo").innerHTML = "Default value and current value is the same: "
        + x.defaultValue + " and " + x.value
        + "<br>Change the month or year of the month field to see the difference!";
    } else {
        document.getElementById("demo").innerHTML = "The default value was: " + defaultVal
        + "<br>The new, current value is: " + currentVal;
    }
}
</script>

</body>
</html>