diff --git a/src/__tests__/waitlist.test.ts b/src/__tests__/waitlist.test.ts
index 383bc80..5c4d209 100644
--- a/src/__tests__/waitlist.test.ts
+++ b/src/__tests__/waitlist.test.ts
@@ -184,6 +184,66 @@ describe("POST /portal/waitlist", () => {
     expect(insertedValues).toHaveLength(1);
   });
 
+  it("normalizes HH:MM:SS preferredTime and returns 201 (GRO-2211)", async () => {
+    selectSessionRow = ACTIVE_SESSION;
+    const res = await jsonRequest("POST", "/portal/waitlist", {
+      petId: VALID_UUID_3,
+      serviceId: VALID_UUID_4,
+      preferredDate: "2026-03-25",
+      preferredTime: "10:00:00",
+    }, { "X-Impersonation-Session-Id": VALID_UUID_5 });
+    expect(res.status).toBe(201);
+    expect(insertedValues[0]?.preferredTime).toBe("10:00:00");
+  });
+
+  it("normalizes HH:MM preferredTime to HH:MM:SS before insert (GRO-2211)", async () => {
+    selectSessionRow = ACTIVE_SESSION;
+    const res = await jsonRequest("POST", "/portal/waitlist", {
+      petId: VALID_UUID_3,
+      serviceId: VALID_UUID_4,
+      preferredDate: "2026-03-25",
+      preferredTime: "10:00",
+    }, { "X-Impersonation-Session-Id": VALID_UUID_5 });
+    expect(res.status).toBe(201);
+    expect(insertedValues[0]?.preferredTime).toBe("10:00:00");
+  });
+
+  it("returns 400 (not 500) for a full ISO datetime preferredTime (GRO-2211)", async () => {
+    selectSessionRow = ACTIVE_SESSION;
+    const res = await jsonRequest("POST", "/portal/waitlist", {
+      petId: VALID_UUID_3,
+      serviceId: VALID_UUID_4,
+      preferredDate: "2026-03-25",
+      preferredTime: "2026-06-09T10:00:00.000Z",
+    }, { "X-Impersonation-Session-Id": VALID_UUID_5 });
+    expect(res.status).toBe(400);
+    expect(insertedValues).toHaveLength(0);
+  });
+
+  it("returns 400 for a malformed preferredDate (GRO-2211)", async () => {
+    selectSessionRow = ACTIVE_SESSION;
+    const res = await jsonRequest("POST", "/portal/waitlist", {
+      petId: VALID_UUID_3,
+      serviceId: VALID_UUID_4,
+      preferredDate: "03/25/2026",
+      preferredTime: "10:00",
+    }, { "X-Impersonation-Session-Id": VALID_UUID_5 });
+    expect(res.status).toBe(400);
+    expect(insertedValues).toHaveLength(0);
+  });
+
+  it("returns 400 for an out-of-range preferredTime (GRO-2211)", async () => {
+    selectSessionRow = ACTIVE_SESSION;
+    const res = await jsonRequest("POST", "/portal/waitlist", {
+      petId: VALID_UUID_3,
+      serviceId: VALID_UUID_4,
+      preferredDate: "2026-03-25",
+      preferredTime: "25:99",
+    }, { "X-Impersonation-Session-Id": VALID_UUID_5 });
+    expect(res.status).toBe(400);
+    expect(insertedValues).toHaveLength(0);
+  });
+
   it("returns 401 without session", async () => {
     const res = await jsonRequest("POST", "/portal/waitlist", {
       petId: VALID_UUID_3,
@@ -258,6 +318,16 @@ describe("PATCH /portal/waitlist/:id", () => {
     expect(updatedValues[0]?.status).toBe("cancelled");
   });
 
+  it("returns 400 (not 500) for a full ISO datetime preferredTime on update (GRO-2211)", async () => {
+    selectSessionRow = ACTIVE_SESSION;
+    selectRows = [WAITLIST_ENTRY];
+    const res = await jsonRequest("PATCH", `/portal/waitlist/${VALID_UUID_1}`, {
+      preferredTime: "2026-06-09T10:00:00.000Z",
+    }, { "X-Impersonation-Session-Id": VALID_UUID_5 });
+    expect(res.status).toBe(400);
+    expect(updatedValues).toHaveLength(0);
+  });
+
   it("returns 401 without session", async () => {
     const res = await jsonRequest("PATCH", `/portal/waitlist/${VALID_UUID_1}`, {
       status: "cancelled",
diff --git a/src/routes/portal.ts b/src/routes/portal.ts
index 8b15f38..aa1593a 100644
--- a/src/routes/portal.ts
+++ b/src/routes/portal.ts
@@ -559,17 +559,33 @@ portalRouter.post("/appointments/:id/cancel", async (c) => {
 
 // ─── Client-facing waitlist routes ────────────────────────────────────────────
 
+// Postgres `date` / `time` columns reject arbitrary strings (e.g. a full ISO
+// datetime), throwing a DateTimeParseError that surfaces as an unhandled 500.
+// Constrain client input here so malformed values are rejected with a 400 by
+// zValidator before they ever reach the DB (GRO-2211 defense-in-depth).
+const preferredDateSchema = z
+  .string()
+  .regex(/^\d{4}-\d{2}-\d{2}$/, "preferredDate must be YYYY-MM-DD");
+const preferredTimeSchema = z
+  .string()
+  .regex(/^([01]\d|2[0-3]):[0-5]\d(:[0-5]\d)?$/, "preferredTime must be HH:MM or HH:MM:SS");
+
+// Normalize HH:MM → HH:MM:SS so it matches the Postgres `time` column format.
+function normalizeTime(value: string): string {
+  return value.length === 5 ? `${value}:00` : value;
+}
+
 const createWaitlistEntrySchema = z.object({
   petId: z.string().uuid(),
   serviceId: z.string().uuid(),
-  preferredDate: z.string(),
-  preferredTime: z.string(),
+  preferredDate: preferredDateSchema,
+  preferredTime: preferredTimeSchema,
 });
 
 const updateWaitlistEntrySchema = z.object({
   status: z.literal("cancelled").optional(),
-  preferredDate: z.string().optional(),
-  preferredTime: z.string().optional(),
+  preferredDate: preferredDateSchema.optional(),
+  preferredTime: preferredTimeSchema.optional(),
 });
 
 portalRouter.post(
@@ -587,7 +603,7 @@ portalRouter.post(
         petId: body.petId,
         serviceId: body.serviceId,
         preferredDate: body.preferredDate,
-        preferredTime: body.preferredTime,
+        preferredTime: normalizeTime(body.preferredTime),
       })
       .returning();
 
@@ -618,7 +634,7 @@ portalRouter.patch(
     const updateData: Record<string, unknown> = { updatedAt: new Date() };
     if (body.status !== undefined) updateData.status = body.status;
     if (body.preferredDate !== undefined) updateData.preferredDate = body.preferredDate;
-    if (body.preferredTime !== undefined) updateData.preferredTime = body.preferredTime;
+    if (body.preferredTime !== undefined) updateData.preferredTime = normalizeTime(body.preferredTime);
 
     const [updated] = await db
       .update(waitlistEntries)