diff --git a/common/clock.cc b/common/clock.cc
index 05c495c..0821a56 100644
--- a/common/clock.cc
+++ b/common/clock.cc
@@ -20,11 +20,11 @@
 
 namespace chromeos_update_engine {
 
-base::Time Clock::GetWallclockTime() {
+base::Time Clock::GetWallclockTime() const {
   return base::Time::Now();
 }
 
-base::Time Clock::GetMonotonicTime() {
+base::Time Clock::GetMonotonicTime() const {
   struct timespec now_ts;
   if (clock_gettime(CLOCK_MONOTONIC_RAW, &now_ts) != 0) {
     // Avoid logging this as an error as call-sites may call this very
@@ -40,7 +40,7 @@
   return base::Time::FromTimeVal(now_tv);
 }
 
-base::Time Clock::GetBootTime() {
+base::Time Clock::GetBootTime() const {
   struct timespec now_ts;
   if (clock_gettime(CLOCK_BOOTTIME, &now_ts) != 0) {
     // Avoid logging this as an error as call-sites may call this very
diff --git a/common/clock.h b/common/clock.h
index 2f373a7..4021fa1 100644
--- a/common/clock.h
+++ b/common/clock.h
@@ -24,13 +24,11 @@
 // Implements a clock.
 class Clock : public ClockInterface {
  public:
-  Clock() {}
+  Clock() = default;
 
-  base::Time GetWallclockTime() override;
-
-  base::Time GetMonotonicTime() override;
-
-  base::Time GetBootTime() override;
+  base::Time GetWallclockTime() const override;
+  base::Time GetMonotonicTime() const override;
+  base::Time GetBootTime() const override;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(Clock);
diff --git a/common/clock_interface.h b/common/clock_interface.h
index 2228983..176505d 100644
--- a/common/clock_interface.h
+++ b/common/clock_interface.h
@@ -32,21 +32,21 @@
   virtual ~ClockInterface() = default;
 
   // Gets the current time e.g. similar to base::Time::Now().
-  virtual base::Time GetWallclockTime() = 0;
+  virtual base::Time GetWallclockTime() const = 0;
 
   // Returns monotonic time since some unspecified starting point. It
   // is not increased when the system is sleeping nor is it affected
   // by NTP or the user changing the time.
   //
   // (This is a simple wrapper around clock_gettime(2) / CLOCK_MONOTONIC_RAW.)
-  virtual base::Time GetMonotonicTime() = 0;
+  virtual base::Time GetMonotonicTime() const = 0;
 
   // Returns monotonic time since some unspecified starting point. It
   // is increased when the system is sleeping but it's not affected
   // by NTP or the user changing the time.
   //
   // (This is a simple wrapper around clock_gettime(2) / CLOCK_BOOTTIME.)
-  virtual base::Time GetBootTime() = 0;
+  virtual base::Time GetBootTime() const = 0;
 };
 
 }  // namespace chromeos_update_engine
diff --git a/common/fake_clock.h b/common/fake_clock.h
index 165ec4d..9c47b57 100644
--- a/common/fake_clock.h
+++ b/common/fake_clock.h
@@ -26,11 +26,11 @@
  public:
   FakeClock() {}
 
-  base::Time GetWallclockTime() override { return wallclock_time_; }
+  base::Time GetWallclockTime() const override { return wallclock_time_; }
 
-  base::Time GetMonotonicTime() override { return monotonic_time_; }
+  base::Time GetMonotonicTime() const override { return monotonic_time_; }
 
-  base::Time GetBootTime() override { return boot_time_; }
+  base::Time GetBootTime() const override { return boot_time_; }
 
   void SetWallclockTime(const base::Time& time) { wallclock_time_ = time; }
 
diff --git a/common/system_state.h b/common/system_state.h
index dc40d36..29b897b 100644
--- a/common/system_state.h
+++ b/common/system_state.h
@@ -21,6 +21,8 @@
 
 #include <base/logging.h>
 
+#include "update_engine/common/clock_interface.h"
+
 namespace chromeos_update_manager {
 
 class UpdateManager;
@@ -39,7 +41,6 @@
 // any circular references in header file inclusion. Hence forward-declaring
 // the required classes.
 class BootControlInterface;
-class ClockInterface;
 class ConnectionManagerInterface;
 class DlcServiceInterface;
 class HardwareInterface;
diff --git a/common/utils.cc b/common/utils.cc
index c8924b1..ac9fa9c 100644
--- a/common/utils.cc
+++ b/common/utils.cc
@@ -52,7 +52,6 @@
 #include <base/strings/stringprintf.h>
 #include <brillo/data_encoding.h>
 
-#include "update_engine/common/clock_interface.h"
 #include "update_engine/common/constants.h"
 #include "update_engine/common/platform_constants.h"
 #include "update_engine/common/prefs_interface.h"