diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5e1115f8b..3af37cf5d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -506,6 +506,7 @@ set(exported_headers ${exported_headers}
                      ${PROJECT_SOURCE_DIR}/include/ua_client_highlevel_async.h)
 
 set(internal_headers ${PROJECT_SOURCE_DIR}/deps/open62541_queue.h
+                     ${PROJECT_SOURCE_DIR}/deps/ziptree.h
                      ${PROJECT_SOURCE_DIR}/deps/pcg_basic.h
                      ${PROJECT_SOURCE_DIR}/deps/libc_time.h
                      ${PROJECT_SOURCE_DIR}/deps/base64.h
diff --git a/include/ua_client.h b/include/ua_client.h
index fb8239686..2dbdb3309 100644
--- a/include/ua_client.h
+++ b/include/ua_client.h
@@ -468,17 +468,35 @@ __UA_Client_AsyncServiceEx(UA_Client *client, const void *request,
                            UA_UInt32 timeout);
 
 /**
- * Repeated Callbacks
- * ------------------
+ * Timed Callbacks
+ * ---------------
  * Repeated callbacks can be attached to a client and will be executed in the
  * defined interval. */
 
 typedef void (*UA_ClientCallback)(UA_Client *client, void *data);
 
+/* Add a callback for execution at a specified time. If the indicated time lies
+ * in the past, then the callback is executed at the next iteration of the
+ * server's main loop.
+ *
+ * @param client The client object.
+ * @param callback The callback that shall be added.
+ * @param data Data that is forwarded to the callback.
+ * @param date The timestamp for the execution time.
+ * @param callbackId Set to the identifier of the repeated callback . This can
+ *        be used to cancel the callback later on. If the pointer is null, the
+ *        identifier is not set.
+ * @return Upon success, UA_STATUSCODE_GOOD is returned. An error code
+ *         otherwise. */
+UA_StatusCode UA_EXPORT
+UA_Client_addTimedCallback(UA_Client *client, UA_ClientCallback callback,
+                           void *data, UA_DateTime date, UA_UInt64 *callbackId);
+
 /* Add a callback for cyclic repetition to the client.
  *
  * @param client The client object.
  * @param callback The callback that shall be added.
+ * @param data Data that is forwarded to the callback.
  * @param interval_ms The callback shall be repeatedly executed with the given
  *        interval (in ms). The interval must be positive. The first execution
  *        occurs at now() + interval at the latest.
@@ -487,19 +505,21 @@ typedef void (*UA_ClientCallback)(UA_Client *client, void *data);
  *        identifier is not set.
  * @return Upon success, UA_STATUSCODE_GOOD is returned. An error code
  *         otherwise. */
-UA_StatusCode
-UA_Client_addRepeatedCallback(UA_Client *client,
-                              UA_ClientCallback callback,
+UA_StatusCode UA_EXPORT
+UA_Client_addRepeatedCallback(UA_Client *client, UA_ClientCallback callback,
                               void *data, UA_Double interval_ms,
                               UA_UInt64 *callbackId);
 
-UA_StatusCode
+UA_StatusCode UA_EXPORT
 UA_Client_changeRepeatedCallbackInterval(UA_Client *client,
                                          UA_UInt64 callbackId,
                                          UA_Double interval_ms);
 
-UA_StatusCode UA_Client_removeRepeatedCallback(UA_Client *client,
-                                               UA_UInt64 callbackId);
+void UA_EXPORT
+UA_Client_removeCallback(UA_Client *client, UA_UInt64 callbackId);
+
+#define UA_Client_removeRepeatedCallback(client, callbackId) \
+    UA_Client_removeCallback(client, callbackId)
 
 /**
  * .. toctree::
diff --git a/include/ua_server.h b/include/ua_server.h
index afa270706..366870497 100644
--- a/include/ua_server.h
+++ b/include/ua_server.h
@@ -77,14 +77,32 @@ UA_StatusCode UA_EXPORT
 UA_Server_run_shutdown(UA_Server *server);
 
 /**
- * Repeated Callbacks
- * ------------------ */
+ * Timed Callbacks
+ * --------------- */
 typedef void (*UA_ServerCallback)(UA_Server *server, void *data);
 
+/* Add a callback for execution at a specified time. If the indicated time lies
+ * in the past, then the callback is executed at the next iteration of the
+ * server's main loop.
+ *
+ * @param server The server object.
+ * @param callback The callback that shall be added.
+ * @param data Data that is forwarded to the callback.
+ * @param date The timestamp for the execution time.
+ * @param callbackId Set to the identifier of the repeated callback . This can
+ *        be used to cancel the callback later on. If the pointer is null, the
+ *        identifier is not set.
+ * @return Upon success, UA_STATUSCODE_GOOD is returned. An error code
+ *         otherwise. */
+UA_StatusCode UA_EXPORT
+UA_Server_addTimedCallback(UA_Server *server, UA_ServerCallback callback,
+                           void *data, UA_DateTime date, UA_UInt64 *callbackId);
+
 /* Add a callback for cyclic repetition to the server.
  *
  * @param server The server object.
  * @param callback The callback that shall be added.
+ * @param data Data that is forwarded to the callback.
  * @param interval_ms The callback shall be repeatedly executed with the given
  *        interval (in ms). The interval must be positive. The first execution
  *        occurs at now() + interval at the latest.
@@ -101,14 +119,15 @@ UA_StatusCode UA_EXPORT
 UA_Server_changeRepeatedCallbackInterval(UA_Server *server, UA_UInt64 callbackId,
                                          UA_Double interval_ms);
 
-/* Remove a repeated callback.
+/* Remove a repeated callback. Does nothing if the callback is not found.
  *
  * @param server The server object.
- * @param callbackId The id of the callback that shall be removed.
- * @return Upon success, UA_STATUSCODE_GOOD is returned.
- *         An error code otherwise. */
-UA_StatusCode UA_EXPORT
-UA_Server_removeRepeatedCallback(UA_Server *server, UA_UInt64 callbackId);
+ * @param callbackId The id of the callback */
+void UA_EXPORT
+UA_Server_removeCallback(UA_Server *server, UA_UInt64 callbackId);
+
+#define UA_Server_removeRepeatedCallback(server, callbackId) \
+    UA_Server_removeCallback(server, callbackId);
 
 /**
  * Reading and Writing Node Attributes
diff --git a/src/client/ua_client.c b/src/client/ua_client.c
index 0f8b7a49a..c6e61370f 100644
--- a/src/client/ua_client.c
+++ b/src/client/ua_client.c
@@ -631,13 +631,18 @@ UA_Client_sendAsyncRequest(UA_Client *client, const void *request,
                                     responseType, userdata, requestId);
 }
 
+UA_StatusCode UA_EXPORT
+UA_Client_addTimedCallback(UA_Client *client, UA_ClientCallback callback,
+                           void *data, UA_DateTime date, UA_UInt64 *callbackId) {
+    return UA_Timer_addTimedCallback(&client->timer, (UA_ApplicationCallback) callback,
+                                     client, data, date, callbackId);
+}
+
 UA_StatusCode
 UA_Client_addRepeatedCallback(UA_Client *client, UA_ClientCallback callback,
-                              void *data, UA_Double interval_ms,
-                              UA_UInt64 *callbackId) {
-    return UA_Timer_addRepeatedCallback(&client->timer,
-                                        (UA_ApplicationCallback) callback, client, data,
-                                        interval_ms, callbackId);
+                              void *data, UA_Double interval_ms, UA_UInt64 *callbackId) {
+    return UA_Timer_addRepeatedCallback(&client->timer, (UA_ApplicationCallback) callback,
+                                        client, data, interval_ms, callbackId);
 }
 
 UA_StatusCode
@@ -647,7 +652,7 @@ UA_Client_changeRepeatedCallbackInterval(UA_Client *client, UA_UInt64 callbackId
                                                    interval_ms);
 }
 
-UA_StatusCode
-UA_Client_removeRepeatedCallback(UA_Client *client, UA_UInt64 callbackId) {
-    return UA_Timer_removeRepeatedCallback(&client->timer, callbackId);
+void
+UA_Client_removeCallback(UA_Client *client, UA_UInt64 callbackId) {
+    UA_Timer_removeCallback(&client->timer, callbackId);
 }
diff --git a/src/pubsub/ua_pubsub.c b/src/pubsub/ua_pubsub.c
index d300e5a32..81ad1a682 100644
--- a/src/pubsub/ua_pubsub.c
+++ b/src/pubsub/ua_pubsub.c
@@ -158,8 +158,7 @@ UA_Server_removeWriterGroup(UA_Server *server, const UA_NodeId writerGroup){
         return UA_STATUSCODE_BADNOTFOUND;
 
     //unregister the publish callback
-    if(UA_PubSubManager_removeRepeatedPubSubCallback(server, wg->publishCallbackId) != UA_STATUSCODE_GOOD)
-        return UA_STATUSCODE_BADINTERNALERROR;
+    UA_PubSubManager_removeRepeatedPubSubCallback(server, wg->publishCallbackId);
 #ifdef UA_ENABLE_PUBSUB_INFORMATIONMODEL
     removeGroupRepresentation(server, wg);
 #endif
diff --git a/src/pubsub/ua_pubsub_manager.c b/src/pubsub/ua_pubsub_manager.c
index 2c0867dca..94ebdf302 100644
--- a/src/pubsub/ua_pubsub_manager.c
+++ b/src/pubsub/ua_pubsub_manager.c
@@ -308,6 +308,7 @@ UA_PubSubManager_delete(UA_Server *server, UA_PubSubManager *pubSubManager) {
 /***********************************/
 /*      PubSub Jobs abstraction    */
 /***********************************/
+
 UA_StatusCode
 UA_PubSubManager_addRepeatedCallback(UA_Server *server, UA_ServerCallback callback,
                                      void *data, UA_Double interval_ms, UA_UInt64 *callbackId) {
@@ -321,9 +322,9 @@ UA_PubSubManager_changeRepeatedCallbackInterval(UA_Server *server, UA_UInt64 cal
     return UA_Timer_changeRepeatedCallbackInterval(&server->timer, callbackId, interval_ms);
 }
 
-UA_StatusCode
+void
 UA_PubSubManager_removeRepeatedPubSubCallback(UA_Server *server, UA_UInt64 callbackId) {
-    return UA_Timer_removeRepeatedCallback(&server->timer, callbackId);
+    UA_Timer_removeCallback(&server->timer, callbackId);
 }
 
 #endif /* UA_ENABLE_PUBSUB */
diff --git a/src/pubsub/ua_pubsub_manager.h b/src/pubsub/ua_pubsub_manager.h
index 9bf86293e..2aa7b9f4b 100644
--- a/src/pubsub/ua_pubsub_manager.h
+++ b/src/pubsub/ua_pubsub_manager.h
@@ -41,7 +41,7 @@ UA_PubSubManager_addRepeatedCallback(UA_Server *server, UA_ServerCallback callba
 UA_StatusCode
 UA_PubSubManager_changeRepeatedCallbackInterval(UA_Server *server, UA_UInt64 callbackId,
                                                 UA_Double interval_ms);
-UA_StatusCode
+void
 UA_PubSubManager_removeRepeatedPubSubCallback(UA_Server *server, UA_UInt64 callbackId);
 
 #endif /* UA_ENABLE_PUBSUB */
diff --git a/src/server/ua_server.c b/src/server/ua_server.c
index 778063452..d0f9ef631 100644
--- a/src/server/ua_server.c
+++ b/src/server/ua_server.c
@@ -258,9 +258,17 @@ UA_Server_new(const UA_ServerConfig *config) {
     return server;
 }
 
-/*****************/
-/* Repeated Jobs */
-/*****************/
+/*******************/
+/* Timed Callbacks */
+/*******************/
+
+UA_StatusCode
+UA_Server_addTimedCallback(UA_Server *server, UA_ServerCallback callback,
+                           void *data, UA_DateTime date, UA_UInt64 *callbackId) {
+    return UA_Timer_addTimedCallback(&server->timer,
+                                     (UA_ApplicationCallback)callback,
+                                     server, data, date, callbackId);
+}
 
 UA_StatusCode
 UA_Server_addRepeatedCallback(UA_Server *server, UA_ServerCallback callback,
@@ -278,9 +286,9 @@ UA_Server_changeRepeatedCallbackInterval(UA_Server *server, UA_UInt64 callbackId
                                                    interval_ms);
 }
 
-UA_StatusCode
-UA_Server_removeRepeatedCallback(UA_Server *server, UA_UInt64 callbackId) {
-    return UA_Timer_removeRepeatedCallback(&server->timer, callbackId);
+void
+UA_Server_removeCallback(UA_Server *server, UA_UInt64 callbackId) {
+    UA_Timer_removeCallback(&server->timer, callbackId);
 }
 
 UA_StatusCode UA_EXPORT
diff --git a/src/server/ua_services_subscription.c b/src/server/ua_services_subscription.c
index 5ed1d1928..11369704d 100644
--- a/src/server/ua_services_subscription.c
+++ b/src/server/ua_services_subscription.c
@@ -34,13 +34,7 @@ setSubscriptionSettings(UA_Server *server, UA_Subscription *subscription,
                         UA_UInt32 requestedMaxKeepAliveCount,
                         UA_UInt32 maxNotificationsPerPublish, UA_Byte priority) {
     /* deregister the callback if required */
-    UA_StatusCode retval = Subscription_unregisterPublishCallback(server, subscription);
-    if(retval != UA_STATUSCODE_GOOD) {
-        UA_LOG_DEBUG_SESSION(&server->config.logger, subscription->session,
-                             "Subscription %u | Could not unregister publish callback with error code %s",
-                             subscription->subscriptionId, UA_StatusCode_name(retval));
-        return retval;
-    }
+    Subscription_unregisterPublishCallback(server, subscription);
 
     /* re-parameterize the subscription */
     subscription->publishingInterval = requestedPublishingInterval;
@@ -61,7 +55,7 @@ setSubscriptionSettings(UA_Server *server, UA_Subscription *subscription,
         subscription->notificationsPerPublish = server->config.maxNotificationsPerPublish;
     subscription->priority = priority;
 
-    retval = Subscription_registerPublishCallback(server, subscription);
+    UA_StatusCode retval = Subscription_registerPublishCallback(server, subscription);
     if(retval != UA_STATUSCODE_GOOD) {
         UA_LOG_DEBUG_SESSION(&server->config.logger, subscription->session,
                              "Subscription %u | Could not register publish callback with error code %s",
diff --git a/src/server/ua_subscription.c b/src/server/ua_subscription.c
index 5dc7b4113..9aa9af4a4 100644
--- a/src/server/ua_subscription.c
+++ b/src/server/ua_subscription.c
@@ -565,20 +565,16 @@ Subscription_registerPublishCallback(UA_Server *server, UA_Subscription *sub) {
     return UA_STATUSCODE_GOOD;
 }
 
-UA_StatusCode
+void
 Subscription_unregisterPublishCallback(UA_Server *server, UA_Subscription *sub) {
     UA_LOG_DEBUG_SESSION(&server->config.logger, sub->session, "Subscription %u | "
                          "Unregister subscription publishing callback", sub->subscriptionId);
 
     if(!sub->publishCallbackIsRegistered)
-        return UA_STATUSCODE_GOOD;
-
-    UA_StatusCode retval = UA_Server_removeRepeatedCallback(server, sub->publishCallbackId);
-    if(retval != UA_STATUSCODE_GOOD)
-        return retval;
+        return;
 
+    UA_Server_removeRepeatedCallback(server, sub->publishCallbackId);
     sub->publishCallbackIsRegistered = false;
-    return UA_STATUSCODE_GOOD;
 }
 
 /* When the session has publish requests stored but the last subscription is
diff --git a/src/server/ua_subscription.h b/src/server/ua_subscription.h
index 66caf72f3..eafef3d66 100644
--- a/src/server/ua_subscription.h
+++ b/src/server/ua_subscription.h
@@ -133,7 +133,7 @@ void UA_MonitoredItem_init(UA_MonitoredItem *mon, UA_Subscription *sub);
 void UA_MonitoredItem_delete(UA_Server *server, UA_MonitoredItem *mon);
 void UA_MonitoredItem_sampleCallback(UA_Server *server, UA_MonitoredItem *mon);
 UA_StatusCode UA_MonitoredItem_registerSampleCallback(UA_Server *server, UA_MonitoredItem *mon);
-UA_StatusCode UA_MonitoredItem_unregisterSampleCallback(UA_Server *server, UA_MonitoredItem *mon);
+void UA_MonitoredItem_unregisterSampleCallback(UA_Server *server, UA_MonitoredItem *mon);
 
 /* Remove entries until mon->maxQueueSize is reached. Sets infobits for lost
  * data if required. */
@@ -212,7 +212,7 @@ struct UA_Subscription {
 UA_Subscription * UA_Subscription_new(UA_Session *session, UA_UInt32 subscriptionId);
 void UA_Subscription_deleteMembers(UA_Server *server, UA_Subscription *sub);
 UA_StatusCode Subscription_registerPublishCallback(UA_Server *server, UA_Subscription *sub);
-UA_StatusCode Subscription_unregisterPublishCallback(UA_Server *server, UA_Subscription *sub);
+void Subscription_unregisterPublishCallback(UA_Server *server, UA_Subscription *sub);
 void UA_Subscription_addMonitoredItem(UA_Subscription *sub, UA_MonitoredItem *newMon);
 UA_MonitoredItem * UA_Subscription_getMonitoredItem(UA_Subscription *sub, UA_UInt32 monitoredItemId);
 
diff --git a/src/server/ua_subscription_monitoreditem.c b/src/server/ua_subscription_monitoreditem.c
index ad174cf7e..c4fa0e796 100644
--- a/src/server/ua_subscription_monitoreditem.c
+++ b/src/server/ua_subscription_monitoreditem.c
@@ -373,12 +373,12 @@ UA_MonitoredItem_registerSampleCallback(UA_Server *server, UA_MonitoredItem *mon
     return retval;
 }
 
-UA_StatusCode
+void
 UA_MonitoredItem_unregisterSampleCallback(UA_Server *server, UA_MonitoredItem *mon) {
     if(!mon->sampleCallbackIsRegistered)
-        return UA_STATUSCODE_GOOD;
+        return;
+    UA_Server_removeRepeatedCallback(server, mon->sampleCallbackId);
     mon->sampleCallbackIsRegistered = false;
-    return UA_Server_removeRepeatedCallback(server, mon->sampleCallbackId);
 }
 
 #endif /* UA_ENABLE_SUBSCRIPTIONS */
diff --git a/src/ua_timer.c b/src/ua_timer.c
index 3e99335b6..2956ddc17 100644
--- a/src/ua_timer.c
+++ b/src/ua_timer.c
@@ -2,87 +2,101 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. 
  *
- *    Copyright 2017 (c) Fraunhofer IOSB (Author: Julius Pfrommer)
+ *    Copyright 2017, 2018 (c) Fraunhofer IOSB (Author: Julius Pfrommer)
  *    Copyright 2017 (c) Stefan Profanter, fortiss GmbH
  */
 
 #include "ua_util_internal.h"
 #include "ua_timer.h"
 
-/* Only one thread operates on the repeated jobs. This is usually the "main"
- * thread with the event loop. All other threads introduce changes via a
- * multi-producer single-consumer (MPSC) queue. The queue is based on a design
- * by Dmitry Vyukov.
- * http://www.1024cores.net/home/lock-free-algorithms/queues/intrusive-mpsc-node-based-queue
- *
- * The RepeatedCallback structure is used both in the sorted list of callbacks
- * and in the MPSC changes queue. For the changes queue, we differentiate
- * between three cases encoded in the callback pointer.
- *
- * callback > 0x01: add the new repeated callback to the sorted list
- * callback == 0x00: remove the callback with the same id
- * callback == 0x01: change the interval of the existing callback */
-
-#define REMOVE_SENTINEL 0x00
-#define CHANGE_SENTINEL 0x01
-
-struct UA_TimerCallbackEntry {
-    SLIST_ENTRY(UA_TimerCallbackEntry) next; /* Next element in the list */
-    UA_DateTime nextTime;                    /* The next time when the callbacks
-                                              * are to be executed */
+struct UA_TimerEntry {
+    ZIP_ENTRY(UA_TimerEntry) zipfields;
+    UA_DateTime nextTime;                    /* The next time when the callback
+                                              * is to be executed */
     UA_UInt64 interval;                      /* Interval in 100ns resolution */
-    UA_UInt64 id;                            /* Id of the repeated callback */
+    UA_Boolean repeated;                     /* Repeated callback? */
 
     UA_ApplicationCallback callback;
     void *application;
     void *data;
+
+    ZIP_ENTRY(UA_TimerEntry) idZipfields;
+    UA_UInt64 id;                            /* Id of the entry */
 };
 
+/* There may be several entries with the same nextTime in the tree. We give them
+ * an absolute order by considering the memory address to break ties. Because of
+ * this, the nextTime property cannot be used to lookup specific entries. */
+static enum ZIP_CMP
+cmpDateTime(const UA_DateTime *a, const UA_DateTime *b) {
+    if(*a < *b)
+        return ZIP_CMP_LESS;
+    if(*a > *b)
+        return ZIP_CMP_MORE;
+    if(a == b)
+        return ZIP_CMP_EQ;
+    if(a < b)
+        return ZIP_CMP_LESS;
+    return ZIP_CMP_MORE;
+}
+
+ZIP_PROTTYPE(UA_TimerZip, UA_TimerEntry, UA_DateTime)
+ZIP_IMPL(UA_TimerZip, UA_TimerEntry, zipfields, UA_DateTime, nextTime, cmpDateTime)
+
+/* The identifiers of entries are unique */
+static enum ZIP_CMP
+cmpId(const UA_UInt64 *a, const UA_UInt64 *b) {
+    if(*a < *b)
+        return ZIP_CMP_LESS;
+    if(*a == *b)
+        return ZIP_CMP_EQ;
+    return ZIP_CMP_MORE;
+}
+
+ZIP_PROTTYPE(UA_TimerIdZip, UA_TimerEntry, UA_UInt64)
+ZIP_IMPL(UA_TimerIdZip, UA_TimerEntry, idZipfields, UA_UInt64, id, cmpId)
+
 void
 UA_Timer_init(UA_Timer *t) {
-    SLIST_INIT(&t->repeatedCallbacks);
-    t->changes_head = (UA_TimerCallbackEntry*)&t->changes_stub;
-    t->changes_tail = (UA_TimerCallbackEntry*)&t->changes_stub;
-    t->changes_stub = NULL;
-    t->idCounter = 0;
+    memset(t, 0, sizeof(UA_Timer));
 }
 
-static void
-enqueueChange(UA_Timer *t, UA_TimerCallbackEntry *tc) {
-    tc->next.sle_next = NULL;
-    UA_TimerCallbackEntry *prev = (UA_TimerCallbackEntry*)
-        UA_atomic_xchg((void * volatile *)&t->changes_head, tc);
-    /* Nothing can be dequeued while the producer is blocked here */
-    prev->next.sle_next = tc; /* Once this change is visible in the consumer,
-                               * the node is dequeued in the following
-                               * iteration */
+static UA_StatusCode
+addCallback(UA_Timer *t, UA_ApplicationCallback callback, void *application, void *data,
+            UA_DateTime nextTime, UA_UInt64 interval, UA_Boolean repeated,
+            UA_UInt64 *callbackId) {
+    /* A callback method needs to be present */
+    if(!callback)
+        return UA_STATUSCODE_BADINTERNALERROR;
+
+    /* Allocate the repeated callback structure */
+    UA_TimerEntry *te = (UA_TimerEntry*)UA_malloc(sizeof(UA_TimerEntry));
+    if(!te)
+        return UA_STATUSCODE_BADOUTOFMEMORY;
+
+    /* Set the repeated callback */
+    te->interval = (UA_UInt64)interval;
+    te->id = ++t->idCounter;
+    te->callback = callback;
+    te->application = application;
+    te->data = data;
+    te->repeated = repeated;
+    te->nextTime = nextTime;
+
+    /* Set the output identifier */
+    if(callbackId)
+        *callbackId = te->id;
+
+    ZIP_INSERT(UA_TimerZip, &t->root, te, ZIP_FFS32(UA_UInt32_random()));
+    ZIP_INSERT(UA_TimerIdZip, &t->idRoot, te, ZIP_RANK(te, zipfields));
+    return UA_STATUSCODE_GOOD;
 }
 
-static UA_TimerCallbackEntry *
-dequeueChange(UA_Timer *t) {
-    UA_TimerCallbackEntry *tail = t->changes_tail;
-    UA_TimerCallbackEntry *next = tail->next.sle_next;
-    if(tail == (UA_TimerCallbackEntry*)&t->changes_stub) {
-        if(!next)
-            return NULL;
-        t->changes_tail = next;
-        tail = next;
-        next = next->next.sle_next;
-    }
-    if(next) {
-        t->changes_tail = next;
-        return tail;
-    }
-    UA_TimerCallbackEntry* head = t->changes_head;
-    if(tail != head)
-        return NULL;
-    enqueueChange(t, (UA_TimerCallbackEntry*)&t->changes_stub);
-    next = tail->next.sle_next;
-    if(next) {
-        t->changes_tail = next;
-        return tail;
-    }
-    return NULL;
+UA_StatusCode
+UA_Timer_addTimedCallback(UA_Timer *t, UA_ApplicationCallback callback,
+                          void *application, void *data, UA_DateTime date,
+                          UA_UInt64 *callbackId) {
+    return addCallback(t, callback, application, data, date, 0, false, callbackId);
 }
 
 /* Adding repeated callbacks: Add an entry with the "nextTime" timestamp in the
@@ -92,70 +106,14 @@ UA_StatusCode
 UA_Timer_addRepeatedCallback(UA_Timer *t, UA_ApplicationCallback callback,
                              void *application, void *data, UA_Double interval_ms,
                              UA_UInt64 *callbackId) {
-    /* A callback method needs to be present */
-    if(!callback)
-        return UA_STATUSCODE_BADINTERNALERROR;
-
     /* The interval needs to be positive */
     if(interval_ms <= 0.0)
         return UA_STATUSCODE_BADINTERNALERROR;
 
-    /* Allocate the repeated callback structure */
-    UA_TimerCallbackEntry *tc =
-        (UA_TimerCallbackEntry*)UA_malloc(sizeof(UA_TimerCallbackEntry));
-    if(!tc)
-        return UA_STATUSCODE_BADOUTOFMEMORY;
-
-    /* Set the repeated callback */
-    tc->interval = (UA_UInt64)(interval_ms * UA_DATETIME_MSEC); /* in 100ns resolution */
-    tc->id = ++t->idCounter;
-    tc->callback = callback;
-    tc->application = application;
-    tc->data = data;
-    tc->nextTime = UA_DateTime_nowMonotonic() + (UA_DateTime)tc->interval;
-
-    /* Set the output identifier */
-    if(callbackId)
-        *callbackId = tc->id;
-
-    /* Enqueue the changes in the MPSC queue */
-    enqueueChange(t, tc);
-    return UA_STATUSCODE_GOOD;
-}
-
-static void
-addTimerCallbackEntry(UA_Timer *t, UA_TimerCallbackEntry * UA_RESTRICT tc) {
-    /* Find the last entry before this callback */
-    UA_TimerCallbackEntry *tmpTc, *afterTc = NULL;
-    SLIST_FOREACH(tmpTc, &t->repeatedCallbacks, next) {
-        if(tmpTc->nextTime >= tc->nextTime)
-            break;
-
-        /* The goal is to have many repeated callbacks with the same repetition
-         * interval in a "block" in order to reduce linear search for re-entry
-         * to the sorted list after processing. Allow the first execution to lie
-         * between "nextTime - 1s" and "nextTime" if this adjustment groups
-         * callbacks with the same repetition interval.
-         * Callbacks of a block are added in reversed order. This design allows
-         * the monitored items of a subscription (if created in a sequence with the
-         * same publish/sample interval) to be executed before the subscription
-         * publish the notifications */
-        if(tmpTc->interval == tc->interval &&
-           tmpTc->nextTime > (tc->nextTime - UA_DATETIME_SEC)) {
-            tc->nextTime = tmpTc->nextTime;
-            break;
-        }
-
-        /* tc is neither in the same interval nor supposed to be executed sooner
-         * than tmpTc. Update afterTc to push tc further back in the timer list. */
-        afterTc = tmpTc;
-    }
-
-    /* Add the repeated callback */
-    if(afterTc)
-        SLIST_INSERT_AFTER(afterTc, tc, next);
-    else
-        SLIST_INSERT_HEAD(&t->repeatedCallbacks, tc, next);
+    UA_UInt64 interval = (UA_UInt64)(interval_ms * UA_DATETIME_MSEC);
+    UA_DateTime nextTime = UA_DateTime_nowMonotonic() + (UA_DateTime)interval;
+    return addCallback(t, callback, application, data, nextTime,
+                       interval, true, callbackId);
 }
 
 UA_StatusCode
@@ -165,211 +123,74 @@ UA_Timer_changeRepeatedCallbackInterval(UA_Timer *t, UA_UInt64 callbackId,
     if(interval_ms <= 0.0)
         return UA_STATUSCODE_BADINTERNALERROR;
 
-    /* Allocate the repeated callback structure */
-    UA_TimerCallbackEntry *tc =
-        (UA_TimerCallbackEntry*)UA_malloc(sizeof(UA_TimerCallbackEntry));
-    if(!tc)
-        return UA_STATUSCODE_BADOUTOFMEMORY;
+    /* Remove from the sorted list */
+    UA_TimerEntry *te = ZIP_FIND(UA_TimerIdZip, &t->idRoot, &callbackId);
+    if(!te)
+        return UA_STATUSCODE_BADNOTFOUND;
 
     /* Set the repeated callback */
-    tc->interval = (UA_UInt64)(interval_ms * UA_DATETIME_MSEC); /* in 100ns resolution */
-    tc->id = callbackId;
-    tc->nextTime = UA_DateTime_nowMonotonic() + (UA_DateTime)tc->interval;
-    tc->callback = (UA_ApplicationCallback)CHANGE_SENTINEL;
-
-    /* Enqueue the changes in the MPSC queue */
-    enqueueChange(t, tc);
+    ZIP_REMOVE(UA_TimerZip, &t->root, te);
+    te->interval = (UA_UInt64)(interval_ms * UA_DATETIME_MSEC); /* in 100ns resolution */
+    te->nextTime = UA_DateTime_nowMonotonic() + (UA_DateTime)te->interval;
+    ZIP_INSERT(UA_TimerZip, &t->root, te, ZIP_RANK(te, zipfields));
     return UA_STATUSCODE_GOOD;
 }
 
-static void
-changeTimerCallbackEntryInterval(UA_Timer *t, UA_UInt64 callbackId,
-                                 UA_UInt64 interval, UA_DateTime nextTime) {
-    /* Remove from the sorted list */
-    UA_TimerCallbackEntry *tc, *prev = NULL;
-    SLIST_FOREACH(tc, &t->repeatedCallbacks, next) {
-        if(callbackId == tc->id) {
-            if(prev)
-                SLIST_REMOVE_AFTER(prev, next);
-            else
-                SLIST_REMOVE_HEAD(&t->repeatedCallbacks, next);
-            break;
-        }
-        prev = tc;
-    }
-    if(!tc)
+void
+UA_Timer_removeCallback(UA_Timer *t, UA_UInt64 callbackId) {
+    UA_TimerEntry *te = ZIP_FIND(UA_TimerIdZip, &t->idRoot, &callbackId);
+    if(!te)
         return;
 
-    /* Adjust settings */
-    tc->interval = interval;
-    tc->nextTime = nextTime;
-
-    /* Reinsert at the new position */
-    addTimerCallbackEntry(t, tc);
-}
-
-/* Removing a repeated callback: Add an entry with the "nextTime" timestamp set
- * to UA_INT64_MAX. The next iteration picks this up and removes the repated
- * callback from the linked list. */
-UA_StatusCode
-UA_Timer_removeRepeatedCallback(UA_Timer *t, UA_UInt64 callbackId) {
-    /* Allocate the repeated callback structure */
-    UA_TimerCallbackEntry *tc =
-        (UA_TimerCallbackEntry*)UA_malloc(sizeof(UA_TimerCallbackEntry));
-    if(!tc)
-        return UA_STATUSCODE_BADOUTOFMEMORY;
-
-    /* Set the repeated callback with the sentinel nextTime */
-    tc->id = callbackId;
-    tc->callback = (UA_ApplicationCallback)REMOVE_SENTINEL;
-
-    /* Enqueue the changes in the MPSC queue */
-    enqueueChange(t, tc);
-    return UA_STATUSCODE_GOOD;
-}
-
-static void
-removeRepeatedCallback(UA_Timer *t, UA_UInt64 callbackId) {
-    UA_TimerCallbackEntry *tc, *prev = NULL;
-    SLIST_FOREACH(tc, &t->repeatedCallbacks, next) {
-        if(callbackId == tc->id) {
-            if(prev)
-                SLIST_REMOVE_AFTER(prev, next);
-            else
-                SLIST_REMOVE_HEAD(&t->repeatedCallbacks, next);
-            UA_free(tc);
-            break;
-        }
-        prev = tc;
-    }
-}
-
-/* Process the changes that were added to the MPSC queue (by other threads) */
-static void
-processChanges(UA_Timer *t) {
-    UA_TimerCallbackEntry *change;
-    while((change = dequeueChange(t))) {
-        switch((uintptr_t)change->callback) {
-        case REMOVE_SENTINEL:
-            removeRepeatedCallback(t, change->id);
-            UA_free(change);
-            break;
-        case CHANGE_SENTINEL:
-            changeTimerCallbackEntryInterval(t, change->id, change->interval,
-                                           change->nextTime);
-            UA_free(change);
-            break;
-        default:
-            addTimerCallbackEntry(t, change);
-        }
-    }
+    ZIP_REMOVE(UA_TimerZip, &t->root, te);
+    ZIP_REMOVE(UA_TimerIdZip, &t->idRoot, te);
+    UA_free(te);
 }
 
 UA_DateTime
 UA_Timer_process(UA_Timer *t, UA_DateTime nowMonotonic,
                  UA_TimerExecutionCallback executionCallback,
                  void *executionApplication) {
-    /* Insert and remove callbacks */
-    processChanges(t);
+    UA_TimerEntry *first;
+    while((first = ZIP_MIN(UA_TimerZip, &t->root)) &&
+          first->nextTime <= nowMonotonic) {
+        ZIP_REMOVE(UA_TimerZip, &t->root, first);
 
-    /* Find the last callback to be executed now */
-    UA_TimerCallbackEntry *firstAfter, *lastNow = NULL;
-    SLIST_FOREACH(firstAfter, &t->repeatedCallbacks, next) {
-        if(firstAfter->nextTime > nowMonotonic)
-            break;
-        lastNow = firstAfter;
-    }
+        /* Reinsert / remove to their new position first. Because the callback
+         * can interact with the zip tree and expects the same entries in the
+         * root and idRoot trees. */
 
-    /* Nothing to do */
-    if(!lastNow) {
-        if(firstAfter)
-            return firstAfter->nextTime;
-        return UA_INT64_MAX;
-    }
-
-    /* Put the callbacks that are executed now in a separate list */
-    UA_TimerCallbackList executedNowList;
-    executedNowList.slh_first = SLIST_FIRST(&t->repeatedCallbacks);
-    lastNow->next.sle_next = NULL;
-
-    /* Fake entry to represent the first element in the newly-sorted list */
-    UA_TimerCallbackEntry tmp_first;
-    tmp_first.nextTime = nowMonotonic - 1; /* never matches for last_dispatched */
-    tmp_first.next.sle_next = firstAfter;
-    UA_TimerCallbackEntry *last_dispatched = &tmp_first;
-
-    /* Iterate over the list of callbacks to process now */
-    UA_TimerCallbackEntry *tc;
-    while((tc = SLIST_FIRST(&executedNowList))) {
-        /* Remove from the list */
-        SLIST_REMOVE_HEAD(&executedNowList, next);
-
-        /* Dispatch/process callback */
-        executionCallback(executionApplication, tc->callback,
-                          tc->application, tc->data);
+        if(!first->repeated) {
+            ZIP_REMOVE(UA_TimerIdZip, &t->idRoot, first);
+            executionCallback(executionApplication, first->callback,
+                              first->application, first->data);
+            UA_free(first);
+            continue;
+        }
 
         /* Set the time for the next execution. Prevent an infinite loop by
          * forcing the next processing into the next iteration. */
-        tc->nextTime += (UA_Int64)tc->interval;
-        if(tc->nextTime < nowMonotonic)
-            tc->nextTime = nowMonotonic + 1;
-
-        /* Find the new position for tc to keep the list sorted */
-        UA_TimerCallbackEntry *prev_tc;
-        if(last_dispatched->nextTime == tc->nextTime) {
-            /* We try to "batch" repeatedCallbacks with the same interval. This
-             * saves a linear search when the last dispatched entry has the same
-             * nextTime timestamp as this entry. */
-            UA_assert(last_dispatched != &tmp_first);
-            prev_tc = last_dispatched;
-        } else {
-            /* Find the position for the next execution by a linear search
-             * starting at last_dispatched or the first element */
-            if(last_dispatched->nextTime < tc->nextTime)
-                prev_tc = last_dispatched;
-            else
-                prev_tc = &tmp_first;
-
-            while(true) {
-                UA_TimerCallbackEntry *n = SLIST_NEXT(prev_tc, next);
-                if(!n || n->nextTime >= tc->nextTime)
-                    break;
-                prev_tc = n;
-            }
-        }
-
-        /* Update last_dispatched to make sure batched callbacks are added in the
-         * same sequence as before they were executed and to save some iterations
-         * of the linear search for callbacks to be added further back in the list. */
-        last_dispatched = tc;
-
-        /* Add entry to the new position in the sorted list */
-        SLIST_INSERT_AFTER(prev_tc, tc, next);
+        first->nextTime += (UA_Int64)first->interval;
+        if(first->nextTime < nowMonotonic)
+            first->nextTime = nowMonotonic + 1;
+        ZIP_INSERT(UA_TimerZip, &t->root, first, ZIP_RANK(first, zipfields));
+        executionCallback(executionApplication, first->callback,
+                          first->application, first->data);
     }
 
-    /* Set the entry-point for the newly sorted list */
-    t->repeatedCallbacks.slh_first = tmp_first.next.sle_next;
+    /* Return the timestamp of the earliest next callback */
+    first = ZIP_MIN(UA_TimerZip, &t->root);
+    return (first) ? first->nextTime : UA_INT64_MAX;
+}
 
-    /* Re-repeat processAddRemoved since one of the callbacks might have removed
-     * or added a callback. So we return a correct timeout. */
-    processChanges(t);
-
-    /* Return timestamp of next repetition */
-    tc = SLIST_FIRST(&t->repeatedCallbacks);
-    if(!tc)
-        return UA_INT64_MAX; /* Main-loop has a max timeout / will continue earlier */
-    return tc->nextTime;
+static void
+freeEntry(UA_TimerEntry *te, void *data) {
+    UA_free(te);
 }
 
 void
 UA_Timer_deleteMembers(UA_Timer *t) {
-    /* Process changes to empty the MPSC queue */
-    processChanges(t);
-
-    /* Remove repeated callbacks */
-    UA_TimerCallbackEntry *current;
-    while((current = SLIST_FIRST(&t->repeatedCallbacks))) {
-        SLIST_REMOVE_HEAD(&t->repeatedCallbacks, next);
-        UA_free(current);
-    }
+    /* Free all nodes and reset the root */
+    ZIP_ITER(UA_TimerZip, &t->root, freeEntry, NULL);
+    ZIP_INIT(&t->root);
 }
diff --git a/src/ua_timer.h b/src/ua_timer.h
index edbe0fde5..220405132 100644
--- a/src/ua_timer.h
+++ b/src/ua_timer.h
@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. 
  *
- *    Copyright 2017 (c) Fraunhofer IOSB (Author: Julius Pfrommer)
+ *    Copyright 2017, 2018 (c) Fraunhofer IOSB (Author: Julius Pfrommer)
  *    Copyright 2017 (c) Stefan Profanter, fortiss GmbH
  */
 
@@ -11,48 +11,37 @@
 
 #include "ua_util_internal.h"
 #include "ua_workqueue.h"
+#include "ziptree.h"
 
 _UA_BEGIN_DECLS
 
-/* An (event) timer triggers callbacks with a recurring interval. Adding,
- * removing and changing repeated callbacks can be done from independent
- * threads. Processing the changes and dispatching callbacks must be done by a
- * single "mainloop" process.
- * Timer callbacks with the same recurring interval are batched into blocks in
- * order to reduce linear search for re-entry to the sorted list after processing.
- * Callbacks are inserted in reversed order (last callback are put first in the block)
- * to allow the monitored items of a subscription (if created in a sequence with the
- * same publish/sample interval) to be executed before the subscription publish the
- * notifications. When callbacks are entered to the timer list after execution they
- * are added in the same order as before execution. */
+struct UA_TimerEntry;
+typedef struct UA_TimerEntry UA_TimerEntry;
 
-/* Forward declaration */
-struct UA_TimerCallbackEntry;
-typedef struct UA_TimerCallbackEntry UA_TimerCallbackEntry;
+ZIP_HEAD(UA_TimerZip, UA_TimerEntry);
+typedef struct UA_TimerZip UA_TimerZip;
 
-/* Linked-list definition */
-typedef SLIST_HEAD(UA_TimerCallbackList, UA_TimerCallbackEntry) UA_TimerCallbackList;
+ZIP_HEAD(UA_TimerIdZip, UA_TimerEntry);
+typedef struct UA_TimerIdZip UA_TimerIdZip;
 
+/* Only for a single thread. Protect by a mutex if required. */
 typedef struct {
-    /* The linked list of callbacks is sorted according to the execution timestamp. */
-    UA_TimerCallbackList repeatedCallbacks;
-
-    /* Changes to the repeated callbacks in a multi-producer single-consumer queue */
-    UA_TimerCallbackEntry * volatile changes_head;
-    UA_TimerCallbackEntry *changes_tail;
-    UA_TimerCallbackEntry *changes_stub;
-
+    UA_TimerZip root; /* The root of the time-sorted zip tree */
+    UA_TimerIdZip idRoot; /* The root of the id-sorted zip tree */
     UA_UInt64 idCounter;
 } UA_Timer;
 
-/* Initialize the Timer. Not thread-safe. */
 void UA_Timer_init(UA_Timer *t);
 
-/* Add a repated callback. Thread-safe, can be used in parallel and in parallel
- * with UA_Timer_process. */
 UA_StatusCode
-UA_Timer_addRepeatedCallback(UA_Timer *t, UA_ApplicationCallback callback, void *application,
-                             void *data, UA_Double interval_ms, UA_UInt64 *callbackId);
+UA_Timer_addTimedCallback(UA_Timer *t, UA_ApplicationCallback callback,
+                          void *application, void *data, UA_DateTime date,
+                          UA_UInt64 *callbackId);
+
+UA_StatusCode
+UA_Timer_addRepeatedCallback(UA_Timer *t, UA_ApplicationCallback callback,
+                             void *application, void *data, UA_Double interval_ms,
+                             UA_UInt64 *callbackId);
 
 /* Change the callback interval. If this is called from within the callback. The
  * adjustment is made during the next _process call. */
@@ -60,10 +49,8 @@ UA_StatusCode
 UA_Timer_changeRepeatedCallbackInterval(UA_Timer *t, UA_UInt64 callbackId,
                                         UA_Double interval_ms);
 
-/* Remove a repated callback. Thread-safe, can be used in parallel and in
- * parallel with UA_Timer_process. */
-UA_StatusCode
-UA_Timer_removeRepeatedCallback(UA_Timer *t, UA_UInt64 callbackId);
+void
+UA_Timer_removeCallback(UA_Timer *t, UA_UInt64 callbackId);
 
 /* Process (dispatch) the repeated callbacks that have timed out. Returns the
  * timestamp of the next scheduled repeated callback. Not thread-safe.
@@ -78,7 +65,6 @@ UA_Timer_process(UA_Timer *t, UA_DateTime nowMonotonic,
                  UA_TimerExecutionCallback executionCallback,
                  void *executionApplication);
 
-/* Remove all repeated callbacks. Not thread-safe. */
 void UA_Timer_deleteMembers(UA_Timer *t);
 
 _UA_END_DECLS