function dateCheck(year, month, day) {
var y = Number(document.getElementsByName(year)[0].value);
var m = Number(document.getElementsByName(month)[0].value);
var day = document.getElementsByName(day)[0];
var d = Number(day.value);
if (y && m) {
var ds = new Date(y, m, 0);
var dsn = Number(ds.getDate());
var html = '<option value="">--日</option>';
for(var i = 1; i <= dsn; i++) {
if (i === d) {
html += '<option value="' + i + '" selected>' + i + '日</option>';
}
else {
html += '<option value="' + i + '">' + i + '日</option>';
}
}
day.innerHTML = html;
}
}