diff --git a/src/base/BUILD.bazel b/src/base/BUILD.bazel
index 3f6d2a624..6af632e87 100644
--- a/src/base/BUILD.bazel
+++ b/src/base/BUILD.bazel
@@ -208,7 +208,6 @@ mozc_cc_library(
     srcs = ["clock.cc"],
     hdrs = ["clock.h"],
     deps = [
-        ":singleton",
         "@com_google_absl//absl/time",
     ],
 )
diff --git a/src/base/clock.cc b/src/base/clock.cc
index 17fe19462..7674faddc 100644
--- a/src/base/clock.cc
+++ b/src/base/clock.cc
@@ -33,7 +33,8 @@
 
 #include "absl/time/clock.h"
 #include "absl/time/time.h"
-#include "base/singleton.h"
+
+#include <atomic>
 
 #if defined(OS_CHROMEOS) || defined(_WIN32)
 constexpr bool kUseAbslLocalTimeZone = false;
@@ -46,6 +47,8 @@ constexpr bool kUseAbslLocalTimeZone = true;
 namespace mozc {
 namespace {
 
+constinit static std::atomic<ClockInterface*> g_mock = nullptr;
+
 absl::TimeZone GetLocalTimeZone() {
   if constexpr (kUseAbslLocalTimeZone) {
     return absl::LocalTimeZone();
@@ -65,27 +68,26 @@ absl::TimeZone GetLocalTimeZone() {
   }
 }
 
-class ClockImpl : public ClockInterface {
- public:
-  ClockImpl() = default;
-  ~ClockImpl() override = default;
-
-  absl::Time GetAbslTime() override { return absl::Now(); }
-
-  absl::TimeZone GetTimeZone() override { return GetLocalTimeZone(); }
-};
 }  // namespace
 
-using ClockSingleton = SingletonMockable<ClockInterface, ClockImpl>;
-
-absl::Time Clock::GetAbslTime() { return ClockSingleton::Get()->GetAbslTime(); }
+absl::Time Clock::GetAbslTime() {
+  if (ClockInterface* mock = g_mock.load(std::memory_order_acquire);
+      mock != nullptr) {
+    return mock->GetAbslTime();
+  }
+  return absl::Now();
+}
 
 absl::TimeZone Clock::GetTimeZone() {
-  return ClockSingleton::Get()->GetTimeZone();
+  if (ClockInterface* mock = g_mock.load(std::memory_order_acquire);
+      mock != nullptr) {
+    return mock->GetTimeZone();
+  }
+  return GetLocalTimeZone();
 }
 
 void Clock::SetClockForUnitTest(ClockInterface* clock) {
-  ClockSingleton::SetMock(clock);
+  g_mock.store(clock, std::memory_order_release);
 }
 
 }  // namespace mozc