Untitled

<!DOCTYPE html>
<html>
    <head>
        <title>Date Selector</title>
    </head>

    <body>
        <script>
            function clear(select) {
                for(var i = select.options.length - 1; i >= 0; i--) {
                    select.remove(i);
                }
            }

            function createMonths() {
                var months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"];
                var select = document.getElementById("month");
                for(var i = 0; i < 12; i++) {
                    var option = document.createElement("option");
                    option.text = months[i];
                    select.add(option);
                }
            }

            function createDays(days) {
                var select = document.getElementById("day");

                var index = select.options.selectedIndex;
                if(index < 0) index = 0;
                if(index >= days) index = days - 1;

                clear(select);
                for(var day = 1; day <= days; day++) {
                    var option = document.createElement("option");
                    option.text = day;
                    select.add(option);
                }

                select.options.selectedIndex = index;
            }

            function createYears(start, end) {
                var select = document.getElementById("year");
                for(var year = end; year >= start; year--) {
                    var option = document.createElement("option");
                    option.text = year;
                    select.add(option);
                }
            }

            function leapYear(year) {
                if(year % 400 == 0) return true;
                if(year % 100 == 0) return false;
                return year % 4;
            }

            function update() {
                var month = document.getElementById("month").options.selectedIndex + 1;

                if(month == 1 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12) {
                    createDays(31);
                }
                else if(month == 4 || month == 6 || month == 9 || month == 11) {
                    createDays(30);
                }
                else {
                    var options = document.getElementById("year").options;
                    var year = options[options.selectedIndex].text;

                    if(leapYear(year)) {
                        createDays(29);
                    }
                    else {
                        createDays(28);
                    }
                }
            }

            window.onload = function() {
                var today = new Date();
                var year = today.getFullYear();

                createMonths();
                createDays(31);
                createYears(year - 100, year);
            };
        </script>

        <select id="month" onchange="update()"></select>
        <select id="day"></select>
        <select id="year" onchange="update()"></select>
    </body>
</html>