Javascript date of birth - prevent selecting 30 Feb etc

I'm fairly inexperienced with Javascript, but I'm trying to create this sort of protection for the registration process so that prevents the user from selecting a date that doesn't exist e.g. September 31 (leap years are anoth开发者_运维百科er matter for February 29 which I'll look into later).

Currently I have this, which only deals with February:

<select name="Day">
<option value="day">Day</option>
<script type="text/javascript">
function printDays(y) {
    var x = 1;
    while (x <= y) {
        document.write("<option value=" + x + ">" + x + "</option>");
        x++;
    }
}
window.onload=printDays(31);
</script>
</select>
<script type="text/javascript">
if (document.getElementById("month").value == feb) {
    printDays(28);
}
</script>

I'm sure there's something fairly obvious that's wrong here, but I'm not experienced enough to pick it out!

Answer has been accepted - although using a date selector is good idea, it does not prevent a determined user (or one with javascript disabled) returning an invalid value.

A partial solution would be to convert the value(s) input to a javascript date then back again. The datum for javascript dates is 1970 - but uses a 64-bit signed value (i.e. has a range of over 500,000 years). Note that in javascript months are numbered 0-11.

But this is not a substitute for serverside validation!

I do nut fully understand your question but consider use of http://www.nickabusey.com/jquery-date-select-boxes-plugin/ to save your work time for something else.

Demo Code

You could always use a plugin such as JQueryUI Date Picker which would handle all of the date rules for you...

Check if year, month and day combination correct

function checkDate(year, month, day) {
    try {
        var d = new Date(year, month, day);
        return !isNaN(d.getDate()) && d.getDate() == day;
    } 
    catch (e) {
        return false;
    } 
}