diff --git a/packages/domain/src/index.ts b/packages/domain/src/index.ts
index 63c0a39ba..c855d1dce 100644
--- a/packages/domain/src/index.ts
+++ b/packages/domain/src/index.ts
@@ -94,17 +94,19 @@ export const eventDraftSchema = z
     ) {
       ctx.addIssue({ code: "custom", message: "End must be after a valid start", path: ["end"] });
     }
-    const dateOnly = /^\d{4}-\d{2}-\d{2}$/;
+    const dateOnly = z.iso.date();
     const timed = /^\d{4}-\d{2}-\d{2}T.*(?:Z|[+-]\d{2}:\d{2})$/;
-    if (
-      !(value.allDay ? dateOnly : timed).test(value.start) ||
-      !(value.allDay ? dateOnly : timed).test(value.end)
-    ) {
+    // Validate the calendar date separately so timed values can retain minute precision.
+    const validTimestamp = (timestamp: string) =>
+      value.allDay
+        ? dateOnly.safeParse(timestamp).success
+        : timed.test(timestamp) && dateOnly.safeParse(timestamp.slice(0, 10)).success;
+    if (!validTimestamp(value.start) || !validTimestamp(value.end)) {
       ctx.addIssue({
         code: "custom",
         message: value.allDay
-          ? "All-day events need date-only values"
-          : "Timed events need an explicit offset",
+          ? "All-day events need valid date-only values"
+          : "Timed events need valid date-times with an explicit offset",
         path: ["start"],
       });
     }
diff --git a/tests/domain.test.ts b/tests/domain.test.ts
index 2d3ba1e91..a397cd912 100644
--- a/tests/domain.test.ts
+++ b/tests/domain.test.ts
@@ -80,6 +80,55 @@ test("events reject an end at or before the start", () => {
   assert.equal(eventDraftSchema.safeParse({ ...event, end: event.start }).success, false);
 });
 
+test("events reject nonexistent dates instead of normalizing them into the next month", () => {
+  for (const allDay of [true, false]) {
+    const format = (date: string) => (allDay ? date : `${date}T10:00:00+08:00`);
+    for (const date of ["2026-02-29", "2026-02-30", "2026-04-31", "2100-02-29"]) {
+      for (const field of ["start", "end"] as const) {
+        assert.equal(
+          eventDraftSchema.safeParse({
+            ...event,
+            allDay,
+            start: format("2000-01-01"),
+            end: format("2200-01-01"),
+            [field]: format(date),
+          }).success,
+          false,
+          `${field} must reject ${format(date)}`,
+        );
+      }
+    }
+  }
+});
+
+test("events accept valid leap days, month ends, and explicit offsets", () => {
+  for (const suffix of [
+    "",
+    "T10:00Z",
+    "T10:00+05:30",
+    "T10:00:00Z",
+    "T10:00:00.123+08:00",
+    "T10:00:00-07:00",
+  ]) {
+    for (const [start, end] of [
+      ["2000-02-29", "2000-03-01"],
+      ["2024-02-29", "2024-03-01"],
+      ["2026-04-30", "2026-05-01"],
+    ]) {
+      assert.equal(
+        eventDraftSchema.safeParse({
+          ...event,
+          allDay: suffix === "",
+          start: start + suffix,
+          end: end + suffix,
+        }).success,
+        true,
+        `${start + suffix} to ${end + suffix} must remain valid`,
+      );
+    }
+  }
+});
+
 test("events default calendarId, location, and description", () => {
   const parsed = eventDraftSchema.parse(event);
   assert.equal(parsed.calendarId, "primary");
diff --git a/tests/google.test.ts b/tests/google.test.ts
index c5a2535f5..667841c89 100644
--- a/tests/google.test.ts
+++ b/tests/google.test.ts
@@ -504,6 +504,26 @@ test("all-day writes use exclusive date-only end and invalid dates never call Go
   assert.equal(requests, 1);
 });
 
+test("calendar creates and updates reject nonexistent dates before contacting Google", async () => {
+  let requests = 0;
+  const client = clientWith(() => {
+    requests++;
+    return json(eventResponse);
+  });
+  for (const allDay of [true, false]) {
+    const suffix = allDay ? "" : "T10:00:00+08:00";
+    const draft = {
+      ...event(),
+      allDay,
+      start: `2026-02-29${suffix}`,
+      end: `2026-03-02${suffix}`,
+    };
+    await assert.rejects(client.createEvent(draft), { name: "ZodError" });
+    await assert.rejects(client.updateEvent("event-1", draft), { name: "ZodError" });
+  }
+  assert.equal(requests, 0);
+});
+
 test("event updates explicitly clear the opposite time representation when switching all-day mode", async () => {
   const client = clientWith(async (request) => {
     if (request.method === "GET") return json(eventResponse);