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diff --git a/src/lib/util/tests/stopwatch_unittest.cc b/src/lib/util/tests/stopwatch_unittest.cc
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+++ b/src/lib/util/tests/stopwatch_unittest.cc
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+// Copyright (C) 2015-2017,2021 Internet Systems Consortium, Inc. ("ISC")
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#include <config.h>
+
+#include <util/stopwatch.h>
+#include <util/stopwatch_impl.h>
+#include <boost/date_time/posix_time/posix_time.hpp>
+#include <gtest/gtest.h>
+#include <unistd.h>
+
+namespace {
+
+using namespace isc;
+using namespace isc::util;
+using namespace boost::posix_time;
+
+/// @brief @c StopwatchImpl mock object.
+///
+/// This class derives from the @c StopwatchImpl to override the
+/// @c StopwatchImpl::getCurrentTime. This method is internally called by
+/// the @c StopwatchImpl to determine the current time. By providing the
+/// implementation of this method which returns the fixed (well known)
+/// timestamp value we can obtain the deterministic values from the accessors
+/// of this class.
+///
+/// This class also includes some convenience methods to return the time
+/// durations in milliseconds.
+class StopwatchMock : public StopwatchImpl {
+public:
+
+    /// @brief Constructor.
+    ///
+    /// @param ref_time Reference time, i.e. the arbitrary time value from
+    /// which time is measured. The @c current_time_ value returned by the
+    /// @c StopwatchMock::getCurrentTime is initialized to this value.
+    /// Subsequent calls to the @c StopwatchMock::ffwd move the value of
+    /// the @c current_time_ forward.
+    StopwatchMock(const ptime& ref_time);
+
+    /// @brief Fast forward time.
+    ///
+    /// Moves the value of the @c current_time_ forward by the specified
+    /// number of milliseconds (microseconds). As a result the timestamp
+    /// returned by the @c StopwatchMock::getCurrentTime moves by this value.
+    /// This simulates the time progress.
+    ///
+    /// @param ms Specifies the number of milliseconds to move current time.
+    /// @param us Specifies the number of fractional microseconds to move
+    /// current time.
+    void ffwd(const uint32_t ms, const uint32_t us = 0);
+
+    /// @brief Returns the last duration in milliseconds.
+    uint32_t getLastDurationInMs() const;
+
+    /// @brief Returns the total duration in milliseconds.
+    uint32_t getTotalDurationInMs() const;
+
+protected:
+
+    /// @brief Returns the current time.
+    ///
+    /// This method returns the fixed @c current_time_ timestamp.
+    virtual ptime getCurrentTime() const;
+
+private:
+
+    /// @brief Holds the current time to be returned by the
+    /// @c StopwatchMock::getCurrentTime.
+    ptime current_time_;
+
+};
+
+StopwatchMock::StopwatchMock(const ptime& ref_time)
+    : StopwatchImpl(), current_time_(ref_time) {
+}
+
+void
+StopwatchMock::ffwd(const uint32_t ms, const uint32_t us) {
+    current_time_ += milliseconds(ms);
+    current_time_ += microseconds(us);
+}
+
+uint32_t
+StopwatchMock::getLastDurationInMs() const {
+    return (getLastDuration().total_milliseconds());
+}
+
+uint32_t
+StopwatchMock::getTotalDurationInMs() const {
+    return (getTotalDuration().total_milliseconds());
+}
+
+ptime
+StopwatchMock::getCurrentTime() const {
+    return (current_time_);
+}
+
+/// @brief Test fixture class for testing @c StopwatchImpl.
+class StopwatchTest : public ::testing::Test {
+public:
+
+    /// @brief Constructor
+    StopwatchTest() = default;
+
+    /// @brief Destructor
+    virtual ~StopwatchTest() = default;
+
+protected:
+
+    /// @brief Set up the test.
+    ///
+    /// Initializes the reference time to be used to create the instances
+    /// of the @c StopwatchMock objects.
+    virtual void SetUp();
+
+    /// @brief Holds the reference time to be used to create the instances
+    /// of the @c StopwatchMock objects.
+    ptime ref_time_;
+};
+
+void
+StopwatchTest::SetUp() {
+    ref_time_ = microsec_clock::universal_time();
+}
+
+/// This test checks the behavior of the stopwatch when it is started
+/// and stopped multiple times. It uses the StopwatchMock object to
+/// control the "time flow" by setting the current time to arbitrary
+/// values using the StopwatchMock::ffwd. In addition, this test
+/// checks that the stopwatch can be reset.
+TEST_F(StopwatchTest, multipleMeasurements) {
+    StopwatchMock stopwatch(ref_time_);
+    // The stopwatch shouldn't automatically start. The initial
+    // durations should be set to 0.
+    EXPECT_EQ(0, stopwatch.getLastDurationInMs());
+    EXPECT_EQ(0, stopwatch.getTotalDurationInMs());
+
+    stopwatch.start();
+
+    // Even though the stopwatch is started, the time is still set to
+    // the initial value. The durations should not be affected.
+    EXPECT_EQ(0, stopwatch.getLastDurationInMs());
+    EXPECT_EQ(0, stopwatch.getTotalDurationInMs());
+
+    // Move the time by 10 ms.
+    stopwatch.ffwd(10);
+
+    // It should be possible to retrieve the durations even when the
+    // stopwatch is running.
+    EXPECT_EQ(10, stopwatch.getLastDurationInMs());
+    EXPECT_EQ(10, stopwatch.getTotalDurationInMs());
+
+    // Now stop it and make sure that the same values are returned.
+    stopwatch.stop();
+
+    EXPECT_EQ(10, stopwatch.getLastDurationInMs());
+    EXPECT_EQ(10, stopwatch.getTotalDurationInMs());
+
+    // Start it again, but don't move the time forward yet.
+    stopwatch.start();
+
+    // The new duration should be 0, but the total should be equal to
+    // the previously measured duration.
+    EXPECT_EQ(0, stopwatch.getLastDurationInMs());
+    EXPECT_EQ(10, stopwatch.getTotalDurationInMs());
+
+    // Move time by 5 ms.
+    stopwatch.ffwd(5);
+
+    // New measured duration should be 5 ms. The total should be 15 ms.
+    EXPECT_EQ(5, stopwatch.getLastDurationInMs());
+    EXPECT_EQ(15, stopwatch.getTotalDurationInMs());
+
+    // Stop it again and make sure the values returned are the same.
+    stopwatch.stop();
+
+    EXPECT_EQ(5, stopwatch.getLastDurationInMs());
+    EXPECT_EQ(15, stopwatch.getTotalDurationInMs());
+
+    // Move the time forward while the stopwatch is stopped.
+    stopwatch.ffwd(8);
+
+    // The measured values should not be affected.
+    EXPECT_EQ(5, stopwatch.getLastDurationInMs());
+    EXPECT_EQ(15, stopwatch.getTotalDurationInMs());
+
+    // Stop should be no-op in this case.
+    stopwatch.stop();
+
+    EXPECT_EQ(5, stopwatch.getLastDurationInMs());
+    EXPECT_EQ(15, stopwatch.getTotalDurationInMs());
+
+    // Start the stopwatch again.
+    stopwatch.start();
+
+    // Move time by 3 ms.
+    stopwatch.ffwd(3);
+
+    // Since the stopwatch is running, the measured duration should
+    // get updated again.
+    EXPECT_EQ(3, stopwatch.getLastDurationInMs());
+    EXPECT_EQ(18, stopwatch.getTotalDurationInMs());
+
+    // Move the time by 2 ms.
+    stopwatch.ffwd(2);
+
+    // Start should be no-op in this case.
+    stopwatch.start();
+
+    // But the durations should be updated.
+    EXPECT_EQ(5, stopwatch.getLastDurationInMs());
+    EXPECT_EQ(20, stopwatch.getTotalDurationInMs());
+
+    // Make sure we can reset.
+    stopwatch.reset();
+
+    EXPECT_EQ(0, stopwatch.getLastDurationInMs());
+    EXPECT_EQ(0, stopwatch.getTotalDurationInMs());
+}
+
+// This test checks that the stopwatch works when the real clock is in use.
+TEST_F(StopwatchTest, realTime) {
+    // Initially, the measured time should be 0.
+    Stopwatch stopwatch;
+    EXPECT_EQ(0, stopwatch.getLastMilliseconds());
+    EXPECT_EQ(0, stopwatch.getTotalMilliseconds());
+
+    // Start the stopwatch.
+    stopwatch.start();
+
+    // Sleep for 1 ms. The stopwatch should measure this duration.
+    usleep(1000);
+
+    stopwatch.stop();
+
+    // The measured duration should be greater or equal 1 ms.
+    long current_duration = stopwatch.getLastMilliseconds();
+    EXPECT_GE(current_duration, 1);
+    EXPECT_EQ(current_duration, stopwatch.getTotalMilliseconds());
+
+    // Sleep for another 2 ms while the stopwatch is in the stopped state.
+    usleep(2000);
+
+    // In the stopped state, we should still have old durations measured.
+    EXPECT_EQ(current_duration, stopwatch.getLastMilliseconds());
+    EXPECT_EQ(current_duration, stopwatch.getTotalMilliseconds());
+
+    // Start it again.
+    stopwatch.start();
+
+    // Sleep for 1 ms.
+    usleep(1000);
+
+    // The durations should get updated as appropriate.
+    EXPECT_GE(stopwatch.getLastMilliseconds(), 1);
+    EXPECT_GE(stopwatch.getTotalMilliseconds(), 2);
+}
+
+// Make sure that we can obtain the durations as microseconds.
+TEST_F(StopwatchTest, getLastMicroseconds) {
+    Stopwatch stopwatch;
+    stopwatch.start();
+
+    usleep(1000);
+
+    stopwatch.stop();
+
+    long current_duration = stopwatch.getLastMicroseconds();
+    EXPECT_GE(current_duration, 1000);
+    EXPECT_EQ(current_duration, stopwatch.getTotalMicroseconds());
+}
+
+// Make sure that we can use the "autostart" option to start the time
+// measurement in the constructor.
+TEST_F(StopwatchTest, autostart) {
+    Stopwatch stopwatch(true);
+    usleep(1000);
+
+    stopwatch.stop();
+
+    EXPECT_GE(stopwatch.getLastMilliseconds(), 1);
+    EXPECT_EQ(stopwatch.getLastMilliseconds(), stopwatch.getTotalMilliseconds());
+}
+
+// Make sure that the conversion to the loggable string works as expected.
+TEST_F(StopwatchTest, logFormat) {
+    time_duration duration = microseconds(223043);
+    EXPECT_EQ("223.043 ms", StopwatchImpl::logFormat(duration));
+
+    duration = microseconds(1234);
+    EXPECT_EQ("1.234 ms", StopwatchImpl::logFormat(duration));
+
+    duration = microseconds(2000);
+    EXPECT_EQ("2.000 ms", StopwatchImpl::logFormat(duration));
+}
+
+} // end of anonymous namespace