Mirror datetime inputs to value attribute

assets/js/date_time_local.mjs

@@ -17,7 +17,7 @@ const DateTimeLocal = {
   },
   syncLocalTime() {
     if (!this.utcTime.value) {
-      this.localTime.value = "";
+      this.setValue(this.localTime, "");
       return;
     }
 
@@ -26,21 +26,33 @@ const DateTimeLocal = {
 
     if (!Number.isNaN(date.getTime())) {
       const pad = (part) => String(part).padStart(2, "0");
-      this.localTime.value =
+      this.setValue(
+        this.localTime,
         `${date.getFullYear()}-${pad(date.getMonth() + 1)}-${pad(date.getDate())}` +
-        `T${pad(date.getHours())}:${pad(date.getMinutes())}`;
+          `T${pad(date.getHours())}:${pad(date.getMinutes())}`,
+      );
     }
   },
   syncUtcTime() {
     if (!this.localTime.value) {
-      this.utcTime.value = "";
+      this.setValue(this.utcTime, "");
       return;
     }
 
     const date = new Date(this.localTime.value);
 
     if (!Number.isNaN(date.getTime())) {
-      this.utcTime.value = date.toISOString().slice(0, 19);
+      this.setValue(this.utcTime, date.toISOString().slice(0, 19));
+      this.setValue(this.localTime, this.localTime.value);
+    }
+  },
+
+  setValue(input, value) {
+    input.value = value;
+    if (value) {
+      input.setAttribute("value", value);
+    } else {
+      input.removeAttribute("value");
     }
   },
   destroyed() {

assets/test/date_time_local_hook_test.mjs

@@ -3,9 +3,27 @@ import test from "node:test";
 
 import DateTimeLocal from "../js/date_time_local.mjs";
 
-function mountHook() {
-  const localTime = { value: "" };
-  const utcTime = { value: "2026-08-04T12:00:00" };
+function makeInput(value = "") {
+  const attributes = new Map();
+  if (value) attributes.set("value", value);
+
+  return {
+    value,
+    setAttribute(name, val) {
+      attributes.set(name, String(val));
+    },
+    removeAttribute(name) {
+      attributes.delete(name);
+    },
+    getAttribute(name) {
+      return attributes.has(name) ? attributes.get(name) : null;
+    },
+  };
+}
+
+function mountHook(utcValue = "2026-08-04T12:00:00") {
+  const localTime = makeInput();
+  const utcTime = makeInput(utcValue);
   const listeners = new Map();
 
   const el = {
@@ -29,6 +47,15 @@ function mountHook() {
   return { el, hook, localTime, utcTime };
 }
 
+function toLocalString(utcValue) {
+  const date = new Date(`${utcValue}Z`);
+  const pad = (part) => String(part).padStart(2, "0");
+  return (
+    `${date.getFullYear()}-${pad(date.getMonth() + 1)}-${pad(date.getDate())}` +
+    `T${pad(date.getHours())}:${pad(date.getMinutes())}`
+  );
+}
+
 test("syncs the hidden UTC value when the datetime control only emits change", () => {
   const { el, localTime, utcTime } = mountHook();
   localTime.value = "2030-01-02T10:30";
@@ -37,3 +64,40 @@ test("syncs the hidden UTC value when the datetime control only emits change", (
 
   assert.equal(utcTime.value, new Date(localTime.value).toISOString().slice(0, 19));
 });
+
+test("mirrors synced values into the value attribute so LiveView sees a DOM diff", () => {
+  const { el, localTime, utcTime } = mountHook();
+
+  assert.equal(localTime.getAttribute("value"), localTime.value);
+
+  localTime.value = "2030-01-02T10:30";
+  el.dispatch("input", localTime);
+
+  assert.equal(utcTime.getAttribute("value"), utcTime.value);
+  assert.equal(localTime.getAttribute("value"), "2030-01-02T10:30");
+});
+
+test("restores the local value after a patch outside the form wipes the input", () => {
+  const utcValue = "2026-08-04T12:00:00";
+  const { hook, localTime } = mountHook(utcValue);
+
+  // Simulate morphdom patching against server HTML that has no value
+  // attribute (e.g. re-render caused by an upload finishing): the input
+  // property is reset and the attribute removed, then `updated` fires.
+  localTime.value = "";
+  localTime.removeAttribute("value");
+  hook.updated();
+
+  assert.equal(localTime.value, toLocalString(utcValue));
+  assert.equal(localTime.getAttribute("value"), toLocalString(utcValue));
+});
+
+test("clears the local value and attribute when the UTC value is empty", () => {
+  const { hook, localTime, utcTime } = mountHook();
+
+  utcTime.value = "";
+  hook.updated();
+
+  assert.equal(localTime.value, "");
+  assert.equal(localTime.getAttribute("value"), null);
+});