diff --git a/packages/domain/src/index.ts b/packages/domain/src/index.ts
index 764ab9c8a..500a18a5d 100644
--- a/packages/domain/src/index.ts
+++ b/packages/domain/src/index.ts
@@ -104,12 +104,13 @@ export const eventDraftSchema = z
       ctx.addIssue({ code: "custom", message: "End must be after a valid start", path: ["end"] });
     }
     const dateOnly = z.iso.date();
-    const timed = /^\d{4}-\d{2}-\d{2}T.*(?:Z|[+-]\d{2}:\d{2})$/;
-    // Validate the calendar date separately so timed values can retain minute precision.
+    const timed = z.union([
+      z.iso.datetime({ offset: true }),
+      z.iso.datetime({ offset: true, precision: -1 }),
+    ]);
+    // Keep minute precision while validating the clock time as well as the date.
     const validTimestamp = (timestamp: string) =>
-      value.allDay
-        ? dateOnly.safeParse(timestamp).success
-        : timed.test(timestamp) && dateOnly.safeParse(timestamp.slice(0, 10)).success;
+      value.allDay ? dateOnly.safeParse(timestamp).success : timed.safeParse(timestamp).success;
     if (!validTimestamp(value.start) || !validTimestamp(value.end)) {
       ctx.addIssue({
         code: "custom",
diff --git a/tests/domain.test.ts b/tests/domain.test.ts
index a397cd912..7c78849d8 100644
--- a/tests/domain.test.ts
+++ b/tests/domain.test.ts
@@ -129,6 +129,25 @@ test("events accept valid leap days, month ends, and explicit offsets", () => {
   }
 });
 
+test("timed events reject 24:00 instead of moving the appointment to the next day", () => {
+  for (const time of ["24:00", "24:00:00", "24:00:00.000"]) {
+    for (const offset of ["Z", "-07:00", "+05:30"]) {
+      for (const field of ["start", "end"] as const) {
+        assert.equal(
+          eventDraftSchema.safeParse({
+            ...event,
+            start: "2000-01-01T10:00:00Z",
+            end: "2200-01-01T10:00:00Z",
+            [field]: `2026-10-10T${time}${offset}`,
+          }).success,
+          false,
+          `${field} must reject ${time}${offset}`,
+        );
+      }
+    }
+  }
+});
+
 test("events default calendarId, location, and description", () => {
   const parsed = eventDraftSchema.parse(event);
   assert.equal(parsed.calendarId, "primary");