diff --git a/src/openlcb/BroadcastTime.hxx b/src/openlcb/BroadcastTime.hxx
index 5b6f54a82..7a8841d2f 100644
--- a/src/openlcb/BroadcastTime.hxx
+++ b/src/openlcb/BroadcastTime.hxx
@@ -50,7 +50,16 @@ class BroadcastTime : public SimpleEventHandler
                     , public TimeBase
 {
 public:
-    typedef std::vector<std::function<void()>>::size_type UpdateSubscribeHandle;
+    /// An opaque data element that is returned from update subscriber
+    /// registration, and allows unregistering a subscriber.
+    typedef size_t UpdateSubscribeHandle;
+
+    /// Callback type used for time update subscribers.
+    ///
+    /// @param old Fast clock's current time according to the pre-update state.
+    /// @param current Fast clock's current time according to the post-update
+    /// state.
+    typedef std::function<void(time_t old, time_t current)> TimeUpdateCallback;
 
     /// Destructor.
     virtual ~BroadcastTime()
@@ -127,7 +136,7 @@ public:
     /// executor.
     /// @param callback function callback to be called.
     /// @return handle to entry that can be used in update_unsubscribe
-    UpdateSubscribeHandle update_subscribe_add(std::function<void()> callback)
+    UpdateSubscribeHandle update_subscribe_add(TimeUpdateCallback callback)
     {
         AtomicHolder h(this);
         for (size_t i = 0; i < callbacks_.size(); ++i)
@@ -135,11 +144,11 @@ public:
             // atempt to garbage collect unused entries
             if (callbacks_[i] == nullptr)
             {
-                callbacks_[i] = callback;
+                callbacks_[i] = std::move(callback);
                 return i;
             }
         }
-        callbacks_.emplace_back(callback);
+        callbacks_.emplace_back(std::move(callback));
         return callbacks_.size() - 1;
     }
 
@@ -225,14 +234,17 @@ protected:
 
     /// Service all of the attached update subscribers. These are called when
     /// there are jumps in time or if the clock is stopped or started.
-    void service_callbacks()
+    /// @param old Fast clock's current time according to the pre-update state.
+    /// @param current Fast clock's current time according to the post-update
+    /// state.
+    void service_callbacks(time_t old, time_t current)
     {
         AtomicHolder h(this);
         for (auto n : callbacks_)
         {
             if (n)
             {
-                n();
+                n(old, current);
             }
         }
     }
@@ -245,7 +257,7 @@ protected:
     StateFlowTimer timer_; ///< timer helper
 
     /// update subscribers
-    std::vector<std::function<void()>> callbacks_;
+    std::vector<TimeUpdateCallback> callbacks_;
 
     int16_t rateRequested_; ///< pending clock rate
 
diff --git a/src/openlcb/BroadcastTimeClient.hxx b/src/openlcb/BroadcastTimeClient.hxx
index 4b834e77a..94037bcce 100644
--- a/src/openlcb/BroadcastTimeClient.hxx
+++ b/src/openlcb/BroadcastTimeClient.hxx
@@ -239,6 +239,7 @@ private:
     void start_stop_logic(bool started)
     {
         bool notify = false;
+        time_t val = 0;
         {
             AtomicHolder h(this);
             if (started_ != started)
@@ -254,12 +255,13 @@ private:
                 }
                 started_ = started;
                 notify = true;
+                val = seconds_;
             }
             // release AtomicHolder
         }
         if (notify)
         {
-            service_callbacks();
+            service_callbacks(val, val);
         }
     }
 
@@ -333,20 +335,17 @@ private:
 
         {
             bool notify = false;
+            time_t old_seconds = 0;
+            time_t new_seconds = 0;
             {
                 AtomicHolder h(this);
-                time_t old_seconds;
+                old_seconds = time();
                 if (rate_ != rateRequested_ ||
                     (immediateUpdate_ && immediatePending_))
                 {
                     // rate changed or an immediate update was pending.
                     notify = true;
                 }
-                else
-                {
-                    // we will need to check for jitter later
-                    old_seconds = time();
-                }
 
                 // we are about to commit an updated time, reset the flags
                 immediateUpdate_ = false;
@@ -355,6 +354,7 @@ private:
                 rate_ = rateRequested_;
                 seconds_ = ::mktime(&tm_);
                 timestamp_ = OSTime::get_monotonic();
+                new_seconds = seconds_;
                 if (rolloverPending_)
                 {
                     // roll forward/back the 3 second delay for the year/date
@@ -377,7 +377,7 @@ private:
             }
             if (notify)
             {
-                service_callbacks();
+                service_callbacks(old_seconds, new_seconds);
             }
         }
 
diff --git a/src/openlcb/BroadcastTimeServer.cxx b/src/openlcb/BroadcastTimeServer.cxx
index 7cb223710..02fcb7bae 100644
--- a/src/openlcb/BroadcastTimeServer.cxx
+++ b/src/openlcb/BroadcastTimeServer.cxx
@@ -543,6 +543,8 @@ class BroadcastTimeServerSet
         bool start_or_stop = false;
         struct tm tm;
         server_->gmtime_r(&tm);
+        time_t old_seconds = server_->time();
+        time_t new_seconds = 0;
 
         uint16_t suffix = message()->data()->suffix_;
 
@@ -590,9 +592,10 @@ class BroadcastTimeServerSet
             AtomicHolder h(server_);
             server_->seconds_ = mktime(&tm);
             server_->timestamp_ = OSTime::get_monotonic();
+            new_seconds = server_->seconds_;
         }
 
-        server_->service_callbacks();
+        server_->service_callbacks(old_seconds, new_seconds);
 
         if (start_or_stop)
         {