diff --git a/src/Challenge.php b/src/Challenge.php
index 4d8797c..d80dc70 100644
--- a/src/Challenge.php
+++ b/src/Challenge.php
@@ -4,6 +4,8 @@
 
 namespace Firehed\WebAuthn;
 
+use DateTimeImmutable;
+
 /**
  * The Challenge object has limited public-facing API:
  * - Create a challenge through the `::random()` method
@@ -55,6 +57,11 @@ public function getBase64Url(): string
         return $this->wrapped->toBase64Url();
     }
 
+    public function getExpiration(): ?DateTimeImmutable
+    {
+        return null;
+    }
+
     /**
      * @return SerializationFormat
      */
diff --git a/src/ChallengeInterface.php b/src/ChallengeInterface.php
index 6375125..fce2b30 100644
--- a/src/ChallengeInterface.php
+++ b/src/ChallengeInterface.php
@@ -4,6 +4,8 @@
 
 namespace Firehed\WebAuthn;
 
+use DateTimeImmutable;
+
 interface ChallengeInterface
 {
     /**
@@ -44,4 +46,17 @@ public function getBase64Url(): string;
      * @internal
      */
     public function getBinary(): BinaryString;
+
+    /**
+     * If non-null, indicates when the challenge should be considered expired.
+     * This should be used in conjunction with request generation and align
+     * with the `timeout` used by `pkOptions`. Be aware that browsers may
+     * override the specified value; the current W3C recommendation (lv3) is
+     * between 5 and 10 minutes (300-600 seconds).
+     *
+     * At present, this is intended as a convenience for storage mechanisms and
+     * expected to be enforced by _ChallengeInterface_ implemetions, not the RP
+     * server and associated internals.
+     */
+    public function getExpiration(): ?DateTimeImmutable;
 }
diff --git a/src/ExpiringChallenge.php b/src/ExpiringChallenge.php
index d45c3b0..e38551a 100644
--- a/src/ExpiringChallenge.php
+++ b/src/ExpiringChallenge.php
@@ -4,7 +4,6 @@
 
 namespace Firehed\WebAuthn;
 
-use DateTimeInterface;
 use DateInterval;
 use DateTimeImmutable;
 use InvalidArgumentException;
@@ -18,13 +17,13 @@
  *
  * @phpstan-type SerializationFormat array{
  *   c: string,
- *   e: int,
+ *   e: numeric-string,
  * }
  */
 class ExpiringChallenge implements ChallengeInterface
 {
     private ChallengeInterface $wrapped;
-    private DateTimeInterface $expiration;
+    private DateTimeImmutable $expiration;
 
     /**
      * @internal
@@ -74,6 +73,11 @@ public function getBinary(): BinaryString
         return $this->wrapped->getBinary();
     }
 
+    public function getExpiration(): DateTimeImmutable
+    {
+        return $this->expiration;
+    }
+
     private function isExpired(): bool
     {
         $diff = $this->expiration->diff(new DateTimeImmutable());
@@ -88,7 +92,7 @@ public function __serialize(): array
     {
         return [
             'c' => $this->wrapped->getBase64(),
-            'e' => $this->expiration->getTimestamp(),
+            'e' => $this->expiration->format('U.u'),
         ];
     }
 
diff --git a/tests/ChallengeInterfaceTestTrait.php b/tests/ChallengeInterfaceTestTrait.php
index 86eb7b3..9e8336d 100644
--- a/tests/ChallengeInterfaceTestTrait.php
+++ b/tests/ChallengeInterfaceTestTrait.php
@@ -30,6 +30,11 @@ public function testSerializationRoundTrip(): void
             $unserialized->getBase64(),
             'Base64 changed',
         );
+
+        self::assertEquals(
+            $challenge->getExpiration(),
+            $unserialized->getExpiration(),
+        );
     }
 
     public function testBinaryMatchesBase64(): void
diff --git a/tests/TestUtilities/TestVectorFixedChallenge.php b/tests/TestUtilities/TestVectorFixedChallenge.php
index b7a4289..a721515 100644
--- a/tests/TestUtilities/TestVectorFixedChallenge.php
+++ b/tests/TestUtilities/TestVectorFixedChallenge.php
@@ -4,6 +4,7 @@
 
 namespace Firehed\WebAuthn\TestUtilities;
 
+use DateTimeImmutable;
 use Exception;
 use Firehed\WebAuthn\{
     BinaryString,
@@ -21,6 +22,11 @@ public function __construct(private string $b64u)
     {
     }
 
+    public function getExpiration(): ?DateTimeImmutable
+    {
+        return null;
+    }
+
     public function getBinary(): BinaryString
     {
         return BinaryString::fromBase64Url($this->b64u);