JavaScript Birth Date Calculator

Inputs Needed

1. Date
2. Month
3. Year

Outputs

How long you have been living.

1. In Months
2. Indays
3. In Minutes

And How many days remaining for your next birthday.

var startyear = "1950";
var endyear = "2010";
var ddlday = document.getElementById("ddlday");
var ddlmonth = document.getElementById("ddlmonth");
var ddlyears = document.getElementById("ddlyears");
var dayop = "";
var monthop = "";
var yearop = "";
//Dropdown for Days
for (var j = 1; j < 32; j++) {
    dayop += "<option value=" + j + ">" + j + "</option>";
}
ddlday.innerHTML = dayop;
//Dropdown for Days
for (var j = 1; j < 13; j++) {
    monthop += "<option value=" + j + ">" + j + "</option>";
}
ddlmonth.innerHTML = monthop;
//DropDown for Year
for (var k = startyear; k < endyear; k++) {
    yearop += "<option value=" + k + ">" + k + "</option>";
}
ddlyears.innerHTML = yearop;

//Check Leap Year

function checkleapyear(datea) {
    if (datea.getYear() % 4 == 0) {
        if (datea.getYear() % 10 != 0) {
            return true;
        } else {
            if (datea.getYear() % 400 == 0)
                return true;
            else
                return false;
        }
    }
    return false;
}
//Calculate Days in Month
function DaysInMonth(Y, M) {
    with(new Date(Y, M, 1, 12)) {
        setDate(0);
        return getDate();
    }
}
//Calculate Date Difference
function datediff(date1, date2) {
    var y1 = date1.getFullYear(),
        m1 = date1.getMonth(),
        d1 = date1.getDate(),
        y2 = date2.getFullYear(),
        m2 = date2.getMonth(),
        d2 = date2.getDate();
    if (d1 < d2) {
        m1--;
        d1 += DaysInMonth(y2, m2);
    }
    if (m1 < m2) {
        y1--;
        m1 += 12;
    }
    return [y1 - y2, m1 - m2, d1 - d2];
}