// Show and hide days according to the selected year and month.
function show_and_hide_days(fp_form) {
    var select_year= $(fp_form).find("select.value_year");
    var select_month= $(fp_form).find("select.value_month");
    var select_day= $(fp_form).find("select.value_day");
    var selected_year= $.parse_int($(select_year).val());
    var selected_month= $.parse_int($(select_month).val());
    var selected_day= $.parse_int($(select_day).val());
    var days_in_month= new Date(selected_year, selected_month, 0).getDate();
    // If the number of days in the selected month is less than 28, change it to 31.
    if (!(days_in_month >= 28))
    {
        days_in_month= 31;
    }
    // If the selected day is bigger than the number of days in the selected month, reduce it to the last day in this month.
    if (selected_day > days_in_month)
    {
        selected_day= days_in_month;
    }
    // Remove days 29 to 31, then append days 29 to days_in_month.
    for (var day= 31; day >= 29; day--)
    {
        $(select_day).find("option[value='" + day +"']").remove();
    }
    for (var day= 29; day <= days_in_month; day++)
    {
        $(select_day).append("<option value="" + day +"">" + day +"</option>");
    }
    // Restore the selected day.
    $(select_day).val(selected_day);
}