diff --git a/.changeset/is-expired-fail-closed.md b/.changeset/is-expired-fail-closed.md
new file mode 100644
index 00000000..d0289698
--- /dev/null
+++ b/.changeset/is-expired-fail-closed.md
@@ -0,0 +1,5 @@
+---
+"@agentcommercekit/vc": patch
+---
+
+Treat an unparseable Verifiable Credential `expirationDate` as expired in `isExpired`, instead of fail-open. Matches the fail-closed stance already used when checking status-list expiry.
diff --git a/packages/vc/README.md b/packages/vc/README.md
index 51aa5d5d..6605643e 100644
--- a/packages/vc/README.md
+++ b/packages/vc/README.md
@@ -132,7 +132,7 @@ the request does not follow redirects, so serve the credential at the URL the
 
 - `verifyParsedCredential(credential, options)` - Verify a credential's proof, expiration, and other claims
 - `verifyProof(proof, resolver)` - Verify a credential's proof
-- `isExpired(credential)` - Check if a credential is expired
+- `isExpired(credential)` - Check if a credential is expired. A present but unparseable `expirationDate` is treated as expired (fail closed).
 - `isRevoked(credential, options)` - Check if a credential has been revoked, against a verified status list credential
 - `parsedJwtCredential(jwt, resolver)` - Parse a JWT credential string into a W3C Credential
 
diff --git a/packages/vc/src/verification/is-expired.test.ts b/packages/vc/src/verification/is-expired.test.ts
index ac456e47..0dd86aac 100644
--- a/packages/vc/src/verification/is-expired.test.ts
+++ b/packages/vc/src/verification/is-expired.test.ts
@@ -47,9 +47,38 @@ describe("isExpired", () => {
     expect(isExpired(credential)).toBe(false)
   })
 
-  it("handles invalid date strings gracefully", () => {
+  it("returns true when expiration date cannot be parsed", () => {
     const credential = buildCredential("invalid-date")
 
+    expect(isExpired(credential)).toBe(true)
+  })
+
+  it("returns true for empty-string expiration dates", () => {
+    const credential = buildCredential("")
+
+    expect(isExpired(credential)).toBe(true)
+  })
+
+  it("returns true for non-ISO numeric strings that Date cannot parse as expiry", () => {
+    const credential = buildCredential("not-a-real-timestamp")
+
+    expect(isExpired(credential)).toBe(true)
+  })
+
+  it("returns true for ISO overflow calendar dates that Date would normalize", () => {
+    // JS Date turns 2099-02-30 into a valid March date; fail closed instead.
+    const credential = buildCredential("2099-02-30T00:00:00.000Z")
+
+    expect(Number.isNaN(new Date("2099-02-30T00:00:00.000Z").getTime())).toBe(
+      false
+    )
+    expect(isExpired(credential)).toBe(true)
+  })
+
+  it("returns false for a future timestamp whose offset crosses a UTC day boundary", () => {
+    // 2099-01-01T00:00:00+14:00 is 2098-12-31 in UTC; calendar fields are still valid.
+    const credential = buildCredential("2099-01-01T00:00:00+14:00")
+
     expect(isExpired(credential)).toBe(false)
   })
 })
diff --git a/packages/vc/src/verification/is-expired.ts b/packages/vc/src/verification/is-expired.ts
index 1d66ac8e..ae42bf9e 100644
--- a/packages/vc/src/verification/is-expired.ts
+++ b/packages/vc/src/verification/is-expired.ts
@@ -1,21 +1,72 @@
 import type { W3CCredential } from "../types"
 
+/**
+ * ISO-8601 timestamps with an explicit calendar date (`YYYY-MM-DD…`).
+ * Used only to reject JS-normalized overflow dates such as `2099-02-30…`.
+ */
+const ISO_CALENDAR_PREFIX = /^(\d{4})-(\d{2})-(\d{2})(?:[Tt ].*)?$/
+
+function daysInMonth(year: number, month: number): number {
+  if (month === 2) {
+    const leap = (year % 4 === 0 && year % 100 !== 0) || year % 400 === 0
+    return leap ? 29 : 28
+  }
+
+  const lengths = [31, 0, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
+  return lengths[month - 1] ?? 0
+}
+
+/**
+ * Return true when `value` looks like an ISO calendar date whose year/month/day
+ * are out of range (e.g. Feb 30). Compares against calendar bounds rather than
+ * UTC fields of the parsed instant, so valid offsets like `+14:00` stay valid.
+ */
+function hasOverflowCalendarDate(value: string): boolean {
+  const match = ISO_CALENDAR_PREFIX.exec(value)
+  if (!match) {
+    return false
+  }
+
+  const year = Number(match[1])
+  const month = Number(match[2])
+  const day = Number(match[3])
+
+  if (month < 1 || month > 12) {
+    return true
+  }
+
+  if (day < 1 || day > daysInMonth(year, month)) {
+    return true
+  }
+
+  return false
+}
+
 /**
  * Check if a credential is expired
  *
+ * Fail closed: a present but unparseable `expirationDate` (including an empty
+ * string) is treated as expired. An attacker must not clear expiry by mangling
+ * the date string. ISO-shaped overflow calendar dates that `Date` would
+ * normalize (e.g. `2099-02-30T00:00:00.000Z`) are also treated as expired.
+ *
  * @param credential - The {@link W3CCredential} to check
- * @returns `true` if the credential is expired, `false` otherwise
+ * @returns `true` if the credential is expired or has an unreadable expiry,
+ *   `false` when there is no expiry or it is still in the future
  */
 export function isExpired(credential: W3CCredential): boolean {
-  if (!credential.expirationDate) {
+  if (credential.expirationDate === undefined) {
     return false
   }
 
+  if (hasOverflowCalendarDate(credential.expirationDate)) {
+    return true
+  }
+
   const expirationDate = new Date(credential.expirationDate)
 
-  if (isNaN(expirationDate.getTime())) {
-    // Expiration date is invalid, so we consider the credential not expired
-    return false
+  if (Number.isNaN(expirationDate.getTime())) {
+    return true
   }
 
   return expirationDate < new Date()
diff --git a/packages/vc/src/verification/is-revoked.ts b/packages/vc/src/verification/is-revoked.ts
index 48255533..c218b330 100644
--- a/packages/vc/src/verification/is-revoked.ts
+++ b/packages/vc/src/verification/is-revoked.ts
@@ -411,9 +411,10 @@ async function resolveStatusListCredential(
     )
   }
 
-  // Check the expiry directly rather than through `isExpired`, which reads an
-  // unparseable date as "not expired". The expiry is the main bound on status
-  // list replay, so a malformed one must not quietly remove it.
+  // Check the expiry directly rather than through `isExpired`, which returns a
+  // boolean and would map an unreadable date to "expired". Status-list fetch
+  // needs a distinct undetermined error so callers can tell malformed expiry
+  // from a list that has genuinely lapsed.
   if (verified.expirationDate !== undefined) {
     const expiresAt = Date.parse(verified.expirationDate)