diff --git a/common/clock_interface.h b/common/clock_interface.h
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+//
+// Copyright (C) 2013 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#ifndef UPDATE_ENGINE_COMMON_CLOCK_INTERFACE_H_
+#define UPDATE_ENGINE_COMMON_CLOCK_INTERFACE_H_
+
+#include <string>
+
+#include <base/time/time.h>
+
+namespace chromeos_update_engine {
+
+// The clock interface allows access to various system clocks. The
+// sole reason for providing this as an interface is unit testing.
+// Additionally, the sole reason for the methods not being static
+// is also unit testing.
+class ClockInterface {
+ public:
+  virtual ~ClockInterface() = default;
+
+  // Gets the current time e.g. similar to base::Time::Now().
+  virtual base::Time GetWallclockTime() = 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;
+
+  // 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;
+};
+
+}  // namespace chromeos_update_engine
+
+#endif  // UPDATE_ENGINE_COMMON_CLOCK_INTERFACE_H_