diff --git a/internal/test/fake_clock.cc b/internal/test/fake_clock.cc
index 418863b6..57c6b29d 100644
--- a/internal/test/fake_clock.cc
+++ b/internal/test/fake_clock.cc
@@ -21,7 +21,10 @@
 
 namespace nearby {
 
-absl::Time FakeClock::Now() const { return now_; }
+absl::Time FakeClock::Now() const {
+  absl::MutexLock lock(&mutex_);
+  return now_;
+}
 
 void FakeClock::AddObserver(absl::string_view name,
                             std::function<void()> observer) {
@@ -36,9 +39,9 @@ void FakeClock::RemoveObserver(absl::string_view name) {
 
 void FakeClock::FastForward(absl::Duration duration) {
   std::vector<std::string> timer_callback_ids;
-  now_ += duration;
   {
     absl::MutexLock lock(&mutex_);
+    now_ += duration;
     for (const auto& observer : observers_) {
       timer_callback_ids.push_back(observer.first);
     }
diff --git a/internal/test/fake_clock.h b/internal/test/fake_clock.h
index dc64aae0..7cef161a 100644
--- a/internal/test/fake_clock.h
+++ b/internal/test/fake_clock.h
@@ -45,8 +45,8 @@ class FakeClock : public Clock {
   int GetObserversCount() ABSL_LOCKS_EXCLUDED(mutex_);
 
  private:
-  absl::Time now_;
   mutable absl::Mutex mutex_;
+  absl::Time now_ ABSL_GUARDED_BY(mutex_);
   absl::flat_hash_map<std::string, std::function<void()>> observers_
       ABSL_GUARDED_BY(mutex_);
 };