diff --git a/service/src/main/java/org/whispersystems/textsecuregcm/experiment/DeviceLastSeenState.java b/service/src/main/java/org/whispersystems/textsecuregcm/experiment/DeviceLastSeenState.java
index f57d16f60..c1750f003 100644
--- a/service/src/main/java/org/whispersystems/textsecuregcm/experiment/DeviceLastSeenState.java
+++ b/service/src/main/java/org/whispersystems/textsecuregcm/experiment/DeviceLastSeenState.java
@@ -3,7 +3,11 @@ package org.whispersystems.textsecuregcm.experiment;
 import javax.annotation.Nullable;
 
 public record DeviceLastSeenState(boolean deviceExists,
-                                  long createdAtMillis,
+                                  // Registration IDs are not guaranteed to be unique across devices and re-registrations.
+                                  // However, for this use case, we accept the possibility of collisions in order to
+                                  // avoid storing plaintext device creation timestamps on the server.
+                                  // This tradeoff is intentional and aligned with our privacy goals.
+                                  int registrationId,
                                   boolean hasPushToken,
                                   long lastSeenMillis,
                                   @Nullable PushTokenType pushTokenType) {
diff --git a/service/src/main/java/org/whispersystems/textsecuregcm/experiment/IdleDevicePushNotificationExperiment.java b/service/src/main/java/org/whispersystems/textsecuregcm/experiment/IdleDevicePushNotificationExperiment.java
index fc3779d29..c44aea9b5 100644
--- a/service/src/main/java/org/whispersystems/textsecuregcm/experiment/IdleDevicePushNotificationExperiment.java
+++ b/service/src/main/java/org/whispersystems/textsecuregcm/experiment/IdleDevicePushNotificationExperiment.java
@@ -4,6 +4,7 @@ import com.google.common.annotations.VisibleForTesting;
 import org.apache.commons.lang3.StringUtils;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
+import org.whispersystems.textsecuregcm.identity.IdentityType;
 import org.whispersystems.textsecuregcm.storage.Account;
 import org.whispersystems.textsecuregcm.storage.Device;
 import org.whispersystems.textsecuregcm.workers.IdleWakeupEligibilityChecker;
@@ -64,7 +65,7 @@ abstract class IdleDevicePushNotificationExperiment implements PushNotificationE
       } else {
         pushTokenType = null;
       }
-      return new DeviceLastSeenState(true, device.getCreated(), hasPushToken(device), device.getLastSeen(), pushTokenType);
+      return new DeviceLastSeenState(true, device.getRegistrationId(IdentityType.ACI), hasPushToken(device), device.getLastSeen(), pushTokenType);
     } else {
       return DeviceLastSeenState.MISSING_DEVICE_STATE;
     }
@@ -116,7 +117,7 @@ abstract class IdleDevicePushNotificationExperiment implements PushNotificationE
 
     assert sample.finalState() != null;
 
-    if (!sample.finalState().deviceExists() || sample.initialState().createdAtMillis() != sample.finalState().createdAtMillis()) {
+    if (!sample.finalState().deviceExists() || sample.initialState().registrationId() != sample.finalState().registrationId()) {
       outcome = Outcome.DELETED;
     } else if (!sample.finalState().hasPushToken()) {
       outcome = Outcome.UNINSTALLED;
diff --git a/service/src/test/java/org/whispersystems/textsecuregcm/experiment/IdleDevicePushNotificationExperimentTest.java b/service/src/test/java/org/whispersystems/textsecuregcm/experiment/IdleDevicePushNotificationExperimentTest.java
index cfc6bdfde..90a899f31 100644
--- a/service/src/test/java/org/whispersystems/textsecuregcm/experiment/IdleDevicePushNotificationExperimentTest.java
+++ b/service/src/test/java/org/whispersystems/textsecuregcm/experiment/IdleDevicePushNotificationExperimentTest.java
@@ -4,6 +4,7 @@ import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.Arguments;
 import org.junit.jupiter.params.provider.MethodSource;
+import org.whispersystems.textsecuregcm.identity.IdentityType;
 import org.whispersystems.textsecuregcm.storage.Account;
 import org.whispersystems.textsecuregcm.storage.Device;
 import reactor.core.publisher.Flux;
@@ -66,37 +67,37 @@ abstract class IdleDevicePushNotificationExperimentTest {
     assertEquals(DeviceLastSeenState.MISSING_DEVICE_STATE, experiment.getState(null, null));
     assertEquals(DeviceLastSeenState.MISSING_DEVICE_STATE, experiment.getState(mock(Account.class), null));
 
-    final long createdAtMillis = CURRENT_TIME.minus(Duration.ofDays(14)).toEpochMilli();
+    final int registrationId = 123;
 
     {
       final Device apnsDevice = mock(Device.class);
       when(apnsDevice.getApnId()).thenReturn("apns-token");
-      when(apnsDevice.getCreated()).thenReturn(createdAtMillis);
+      when(apnsDevice.getRegistrationId(IdentityType.ACI)).thenReturn(registrationId);
       when(apnsDevice.getLastSeen()).thenReturn(CURRENT_TIME.toEpochMilli());
 
       assertEquals(
-          new DeviceLastSeenState(true, createdAtMillis, true, CURRENT_TIME.toEpochMilli(), DeviceLastSeenState.PushTokenType.APNS),
+          new DeviceLastSeenState(true, registrationId, true, CURRENT_TIME.toEpochMilli(), DeviceLastSeenState.PushTokenType.APNS),
           experiment.getState(mock(Account.class), apnsDevice));
     }
 
     {
       final Device fcmDevice = mock(Device.class);
       when(fcmDevice.getGcmId()).thenReturn("fcm-token");
-      when(fcmDevice.getCreated()).thenReturn(createdAtMillis);
+      when(fcmDevice.getRegistrationId(IdentityType.ACI)).thenReturn(registrationId);
       when(fcmDevice.getLastSeen()).thenReturn(CURRENT_TIME.toEpochMilli());
 
       assertEquals(
-          new DeviceLastSeenState(true, createdAtMillis, true, CURRENT_TIME.toEpochMilli(), DeviceLastSeenState.PushTokenType.FCM),
+          new DeviceLastSeenState(true, registrationId, true, CURRENT_TIME.toEpochMilli(), DeviceLastSeenState.PushTokenType.FCM),
           experiment.getState(mock(Account.class), fcmDevice));
     }
 
     {
       final Device noTokenDevice = mock(Device.class);
-      when(noTokenDevice.getCreated()).thenReturn(createdAtMillis);
+      when(noTokenDevice.getRegistrationId(IdentityType.ACI)).thenReturn(registrationId);
       when(noTokenDevice.getLastSeen()).thenReturn(CURRENT_TIME.toEpochMilli());
 
       assertEquals(
-          new DeviceLastSeenState(true, createdAtMillis, false, CURRENT_TIME.toEpochMilli(), null),
+          new DeviceLastSeenState(true, registrationId, false, CURRENT_TIME.toEpochMilli(), null),
           experiment.getState(mock(Account.class), noTokenDevice));
     }
   }
@@ -150,10 +151,10 @@ abstract class IdleDevicePushNotificationExperimentTest {
             IdleDevicePushNotificationExperiment.Outcome.DELETED
         ),
 
-        // Device re-registered (i.e. "created" timestamp changed)
+        // Device re-registered (i.e. registration ID changed)
         Arguments.of(
-            new DeviceLastSeenState(true, 0, true, 0, DeviceLastSeenState.PushTokenType.APNS),
-            new DeviceLastSeenState(true, 1, true, 1, DeviceLastSeenState.PushTokenType.APNS),
+            new DeviceLastSeenState(true, 123, true, 0, DeviceLastSeenState.PushTokenType.APNS),
+            new DeviceLastSeenState(true, 1234, true, 1, DeviceLastSeenState.PushTokenType.APNS),
             IdleDevicePushNotificationExperiment.Outcome.DELETED
         ),