diff --git a/packages/ack-pay/src/schemas.test.ts b/packages/ack-pay/src/schemas.test.ts
new file mode 100644
index 0000000..8d931c0
--- /dev/null
+++ b/packages/ack-pay/src/schemas.test.ts
@@ -0,0 +1,51 @@
+import * as v from "valibot"
+import { describe, expect, it } from "vitest"
+
+import { paymentRequestSchema as valibotPaymentRequestSchema } from "./schemas/valibot"
+import { paymentRequestSchema as zodV3PaymentRequestSchema } from "./schemas/zod/v3"
+import { paymentRequestSchema as zodV4PaymentRequestSchema } from "./schemas/zod/v4"
+
+const paymentRequest = {
+  id: "test-payment-request-id",
+  paymentOptions: [
+    {
+      id: "test-payment-option-id",
+      amount: 10,
+      decimals: 2,
+      currency: "USD",
+      recipient: "sol:123",
+    },
+  ],
+}
+
+describe("paymentRequestSchema", () => {
+  it("reports invalid expiresAt strings as schema errors", () => {
+    const input = {
+      ...paymentRequest,
+      expiresAt: "invalid-date",
+    }
+
+    expect(v.safeParse(valibotPaymentRequestSchema, input).success).toBe(false)
+    expect(zodV3PaymentRequestSchema.safeParse(input).success).toBe(false)
+    expect(zodV4PaymentRequestSchema.safeParse(input).success).toBe(false)
+  })
+
+  it("normalizes valid expiresAt inputs to an ISO string", () => {
+    const expected = "2024-12-31T23:59:59.000Z"
+    for (const expiresAt of [
+      new Date("2024-12-31T23:59:59Z"),
+      "2024-12-31T23:59:59Z",
+    ]) {
+      const input = { ...paymentRequest, expiresAt }
+
+      const valibot = v.safeParse(valibotPaymentRequestSchema, input)
+      expect(valibot.success && valibot.output.expiresAt).toBe(expected)
+
+      const v3 = zodV3PaymentRequestSchema.safeParse(input)
+      expect(v3.success && v3.data.expiresAt).toBe(expected)
+
+      const v4 = zodV4PaymentRequestSchema.safeParse(input)
+      expect(v4.success && v4.data.expiresAt).toBe(expected)
+    }
+  })
+})
diff --git a/packages/ack-pay/src/schemas/valibot.ts b/packages/ack-pay/src/schemas/valibot.ts
index 06a58f1..d97ed94 100644
--- a/packages/ack-pay/src/schemas/valibot.ts
+++ b/packages/ack-pay/src/schemas/valibot.ts
@@ -4,6 +4,12 @@ import * as v from "valibot"
 
 const urlOrDidUri = v.union([v.pipe(v.string(), v.url()), didUriSchema])
 
+const timestampSchema = v.pipe(
+  v.union([v.date(), v.string()]),
+  v.check((input) => !Number.isNaN(new Date(input).getTime()), "Invalid date"),
+  v.transform((input) => new Date(input).toISOString()),
+)
+
 export const paymentOptionSchema = v.object({
   id: v.string(),
   amount: v.union([v.pipe(v.number(), v.integer(), v.gtValue(0)), v.string()]),
@@ -19,12 +25,7 @@ export const paymentRequestSchema = v.object({
   id: v.string(),
   description: v.optional(v.string()),
   serviceCallback: v.optional(v.pipe(v.string(), v.url())),
-  expiresAt: v.optional(
-    v.pipe(
-      v.union([v.date(), v.string()]),
-      v.transform((input) => new Date(input).toISOString()),
-    ),
-  ),
+  expiresAt: v.optional(timestampSchema),
   paymentOptions: v.pipe(
     v.tupleWithRest([paymentOptionSchema], paymentOptionSchema),
     v.nonEmpty(),
diff --git a/packages/ack-pay/src/schemas/zod/v3.ts b/packages/ack-pay/src/schemas/zod/v3.ts
index 2e3ee83..36acd31 100644
--- a/packages/ack-pay/src/schemas/zod/v3.ts
+++ b/packages/ack-pay/src/schemas/zod/v3.ts
@@ -4,6 +4,21 @@ import { z } from "zod/v3"
 
 const urlOrDidUri = z.union([z.string().url(), didUriSchema])
 
+const timestampSchema = z
+  .union([z.date(), z.string()])
+  .transform((val, ctx) => {
+    const date = new Date(val)
+    if (Number.isNaN(date.getTime())) {
+      ctx.addIssue({
+        code: z.ZodIssueCode.custom,
+        message: "Invalid date",
+      })
+      return z.NEVER
+    }
+
+    return date.toISOString()
+  })
+
 export const paymentOptionSchema = z.object({
   id: z.string(),
   amount: z.union([z.number().int().positive(), z.string()]),
@@ -19,10 +34,7 @@ export const paymentRequestSchema = z.object({
   id: z.string(),
   description: z.string().optional(),
   serviceCallback: z.string().url().optional(),
-  expiresAt: z
-    .union([z.date(), z.string()])
-    .transform((val) => new Date(val).toISOString())
-    .optional(),
+  expiresAt: timestampSchema.optional(),
   paymentOptions: z.array(paymentOptionSchema).nonempty(),
 })
 
diff --git a/packages/ack-pay/src/schemas/zod/v4.ts b/packages/ack-pay/src/schemas/zod/v4.ts
index 55c19f0..bdd0374 100644
--- a/packages/ack-pay/src/schemas/zod/v4.ts
+++ b/packages/ack-pay/src/schemas/zod/v4.ts
@@ -4,6 +4,22 @@ import * as z from "zod/v4"
 
 const urlOrDidUri = z.union([z.url(), didUriSchema])
 
+const timestampSchema = z
+  .union([z.date(), z.string()])
+  .transform((val, ctx) => {
+    const date = new Date(val)
+    if (Number.isNaN(date.getTime())) {
+      ctx.addIssue({
+        code: "custom",
+        message: "Invalid date",
+        input: val,
+      })
+      return z.NEVER
+    }
+
+    return date.toISOString()
+  })
+
 export const paymentOptionSchema = z.object({
   id: z.string(),
   amount: z.union([z.number().int().positive(), z.string()]),
@@ -19,10 +35,7 @@ export const paymentRequestSchema = z.object({
   id: z.string(),
   description: z.string().optional(),
   serviceCallback: z.url().optional(),
-  expiresAt: z
-    .union([z.date(), z.string()])
-    .transform((val) => new Date(val).toISOString())
-    .optional(),
+  expiresAt: timestampSchema.optional(),
   paymentOptions: z.array(paymentOptionSchema).nonempty(),
 })