--- /dev/null
+<?php
+
+namespace Drupal\Tests\Component\Utility;
+
+use Drupal\Tests\UnitTestCase;
+use Drupal\Component\Utility\Timer;
+
+/**
+ * Tests the Timer system.
+ *
+ * @group Utility
+ *
+ * @coversDefaultClass \Drupal\Component\Utility\Timer
+ */
+class TimerTest extends UnitTestCase {
+
+ /**
+ * Tests Timer::read() time accumulation accuracy across multiple restarts.
+ *
+ * @covers ::start
+ * @covers ::stop
+ * @covers ::read
+ */
+ public function testTimer() {
+ Timer::start('test');
+ usleep(5000);
+ $value = Timer::read('test');
+ usleep(5000);
+ $value2 = Timer::read('test');
+ usleep(5000);
+ $value3 = Timer::read('test');
+ usleep(5000);
+ $value4 = Timer::read('test');
+
+ // Although we sleep for 5 milliseconds, we should test that at least 4 ms
+ // have past because usleep() is not reliable on Windows. See
+ // http://php.net/manual/function.usleep.php for more information. The
+ // purpose of the test to validate that the Timer class can measure elapsed
+ // time not the granularity of usleep() on a particular OS.
+ $this->assertGreaterThanOrEqual(4, $value, 'Timer failed to measure at least 4 milliseconds of sleeping while running.');
+
+ $this->assertGreaterThanOrEqual($value + 4, $value2, 'Timer failed to measure at least 8 milliseconds of sleeping while running.');
+
+ $this->assertGreaterThanOrEqual($value2 + 4, $value3, 'Timer failed to measure at least 12 milliseconds of sleeping while running.');
+
+ $this->assertGreaterThanOrEqual($value3 + 4, $value4, 'Timer failed to measure at least 16 milliseconds of sleeping while running.');
+
+ // Stop the timer.
+ $value5 = Timer::stop('test');
+ $this->assertGreaterThanOrEqual($value4, $value5['time'], 'Timer measured after stopping was not greater than last measurement.');
+
+ // Read again.
+ $value6 = Timer::read('test');
+ $this->assertEquals($value5['time'], $value6, 'Timer measured after stopping was not equal to the stopped time.');
+
+ // Restart.
+ Timer::start('test');
+ usleep(5000);
+ $value7 = Timer::read('test');
+ $this->assertGreaterThanOrEqual($value6 + 4, $value7, 'Timer failed to measure at least 16 milliseconds of sleeping while running.');
+
+ // Stop again.
+ $value8 = Timer::stop('test');
+ $value9 = Timer::read('test');
+ $this->assertEquals($value8['time'], $value9, 'Timer measured after stopping not equal to stop time.');
+ }
+
+}