diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/mls/group/MlsGroup.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/mls/group/MlsGroup.kt
index 083cf18c08..710089098f 100644
--- a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/mls/group/MlsGroup.kt
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/mls/group/MlsGroup.kt
@@ -2001,7 +2001,7 @@ class MlsGroup private constructor(
             // the signer authorized.
             val lifetime = updatePath.leafNode.lifetime
             if (lifetime != null) {
-                val now = TimeUtils.now()
+                val now = policy.now()
                 require(now >= lifetime.notBefore && now <= lifetime.notAfter) {
                     "LeafNode lifetime expired or not yet valid in UpdatePath"
                 }
@@ -2809,7 +2809,7 @@ class MlsGroup private constructor(
         // Validate lifetime
         val lifetime = leafNode.lifetime
         if (lifetime != null) {
-            val now = TimeUtils.now()
+            val now = policy.now()
             require(now >= lifetime.notBefore && now <= lifetime.notAfter) {
                 "KeyPackage lifetime expired or not yet valid"
             }
diff --git a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/mls/group/MlsGroupPolicy.kt b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/mls/group/MlsGroupPolicy.kt
index 33ea399729..092c66143d 100644
--- a/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/mls/group/MlsGroupPolicy.kt
+++ b/quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/mls/group/MlsGroupPolicy.kt
@@ -23,6 +23,7 @@ package com.vitorpamplona.quartz.mls.group
 import com.vitorpamplona.quartz.mls.messages.CommitResult
 import com.vitorpamplona.quartz.mls.tree.Capabilities
 import com.vitorpamplona.quartz.mls.tree.Extension
+import com.vitorpamplona.quartz.utils.TimeUtils
 
 /**
  * The application's rules about who in a group may do what.
@@ -128,6 +129,14 @@ interface MlsGroupPolicy {
      */
     val knownExtensionTypes: Set<Int> get() = emptySet()
 
+    /**
+     * The clock, in unix seconds, that LeafNode lifetimes (RFC 9420 §7.2) are
+     * checked against. Wall time in production; a test pins it to a moment
+     * inside a recorded fixture's validity window, so fixtures signed by
+     * another implementation don't expire out from under the suite.
+     */
+    fun now(): Long = TimeUtils.now()
+
     companion object {
         /** RFC 9420 exactly as written: any member may commit anything valid. */
         val Permissive: MlsGroupPolicy = object : MlsGroupPolicy {}
diff --git a/quartz/src/jvmAndroidTest/kotlin/com/vitorpamplona/quartz/cordn/interop/CordnLifecycleInteropTest.kt b/quartz/src/jvmAndroidTest/kotlin/com/vitorpamplona/quartz/cordn/interop/CordnLifecycleInteropTest.kt
index 5915d75df1..20f07d40f5 100644
--- a/quartz/src/jvmAndroidTest/kotlin/com/vitorpamplona/quartz/cordn/interop/CordnLifecycleInteropTest.kt
+++ b/quartz/src/jvmAndroidTest/kotlin/com/vitorpamplona/quartz/cordn/interop/CordnLifecycleInteropTest.kt
@@ -260,7 +260,7 @@ class CordnLifecycleInteropTest {
         val group =
             MlsGroup.create(
                 identity = CordnCredential.of(alice).identity,
-                policy = CordnGroupPolicy,
+                policy = TsMlsFixtures.cordnPolicy,
                 initialExtensions = listOf(CordnGroupMetadata(name = "from Kotlin").toExtension()),
             )
 
diff --git a/quartz/src/jvmAndroidTest/kotlin/com/vitorpamplona/quartz/cordn/interop/KotlinArtifactProducerTest.kt b/quartz/src/jvmAndroidTest/kotlin/com/vitorpamplona/quartz/cordn/interop/KotlinArtifactProducerTest.kt
index 73c726fe72..b50583a525 100644
--- a/quartz/src/jvmAndroidTest/kotlin/com/vitorpamplona/quartz/cordn/interop/KotlinArtifactProducerTest.kt
+++ b/quartz/src/jvmAndroidTest/kotlin/com/vitorpamplona/quartz/cordn/interop/KotlinArtifactProducerTest.kt
@@ -83,7 +83,7 @@ class KotlinArtifactProducerTest {
         val group =
             MlsGroup.create(
                 identity = CordnCredential.of(alice).identity,
-                policy = CordnGroupPolicy,
+                policy = TsMlsFixtures.cordnPolicy,
                 initialExtensions = listOf(metadata.toExtension()),
             )
         write("k-alice.pk", alice)
diff --git a/quartz/src/jvmAndroidTest/kotlin/com/vitorpamplona/quartz/cordn/interop/TsMlsFixtures.kt b/quartz/src/jvmAndroidTest/kotlin/com/vitorpamplona/quartz/cordn/interop/TsMlsFixtures.kt
index 5628e53cd9..f915bd36bd 100644
--- a/quartz/src/jvmAndroidTest/kotlin/com/vitorpamplona/quartz/cordn/interop/TsMlsFixtures.kt
+++ b/quartz/src/jvmAndroidTest/kotlin/com/vitorpamplona/quartz/cordn/interop/TsMlsFixtures.kt
@@ -20,9 +20,11 @@
  */
 package com.vitorpamplona.quartz.cordn.interop
 
+import com.vitorpamplona.quartz.cordn.groups.CordnGroupPolicy
 import com.vitorpamplona.quartz.mls.codec.TlsReader
 import com.vitorpamplona.quartz.mls.codec.TlsWriter
 import com.vitorpamplona.quartz.mls.crypto.Ed25519
+import com.vitorpamplona.quartz.mls.group.MlsGroupPolicy
 import com.vitorpamplona.quartz.mls.messages.KeyPackageBundle
 import com.vitorpamplona.quartz.mls.messages.MlsKeyPackage
 import kotlin.io.encoding.Base64
@@ -43,6 +45,19 @@ object TsMlsFixtures {
     fun b64(name: String): ByteArray = Base64.decode(text(name))
 
     fun hex(name: String): ByteArray = text(name).chunked(2).map { it.toInt(16).toByte() }.toByteArray()
+
+    /**
+     * 2026-09-21T09:46:40Z, inside the fixtures' KeyPackage lifetime
+     * (2026-09-17T03:34:16Z to 2026-10-03T08:34:16Z). ts-mls signed those
+     * lifetimes, so they cannot be extended here; the clock moves instead.
+     */
+    const val NOW: Long = 1_790_000_000L
+
+    /** [CordnGroupPolicy] with its clock pinned to [NOW]. */
+    val cordnPolicy: MlsGroupPolicy =
+        object : MlsGroupPolicy by CordnGroupPolicy {
+            override fun now(): Long = NOW
+        }
 }
 
 /**