diff --git a/src/js/calendar.js b/src/js/calendar.js
new file mode 100644
index 0000000..920bf98
--- /dev/null
+++ b/src/js/calendar.js
@@ -0,0 +1,8 @@
+document.addEventListener("DOMContentLoaded", _ => {
+  document.querySelectorAll("select").forEach(el => {
+    el.addEventListener("change", _ => {
+      el.setAttribute("data-value", el.value);
+    });
+    el.setAttribute("data-value", el.value);
+  });
+});