add date placeholder

app/javascript/elements/date_placeholder.js

@@ -0,0 +1,10 @@
+export default class extends HTMLElement {
+  connectedCallback () {
+    const input = this.querySelector('input')
+    const placeholder = this.querySelector('span')
+
+    input.addEventListener('change', () => {
+      placeholder.classList.toggle('hidden', !!input.value)
+    })
+  }
+}