javascript Nan error in Firefox & opera & in IE work Properly

function ageCount() {
    debugger;
    var age = 0;
    var AgeOF = 0;
    var date1 = new Date();
    var dob = document.getElementById('MainContent_txtDOB').value;
    var date2 = new Date(dob);
    var pattern = /^(0?[1-9]|[12][0-9]|3[01])[\/\-](0?[1-9]|1[012])[\/\-]\d{4}$/; //Regex to validate date format (dd/mm/yyyy)
    if (pattern.test(dob)) {
        var y1 = date1.getFullYear(); //getting current year
        var y2 = date2.getFullYear(); //getting dob year
       age = parseInt(y1,10) - parseInt(y2,10);           //calculating age 
       //document.write("Age : " + age);is
       AgeOF = parseInt(age, 10);
       if(isNaN(AgeOF))
        {
            AgeOF = 0;
        }
        alert(AgeOF);

        document.getElementById('MainContent_txtAge').value = AgeOF;
        return true;
    } else {
        alert("Invalid date format. Please Input in (dd/mm/yyyy) format!");
        return false;
    }
}