diff --git a/tests/time_test.cpp b/tests/time_test.cpp
index c631b6a..d2fec9a 100644
--- a/tests/time_test.cpp
+++ b/tests/time_test.cpp
@@ -404,3 +404,11 @@
   ASSERT_EQ(0, ts2.tv_sec);
   ASSERT_LT(ts2.tv_nsec, 1000000);
 }
+
+TEST(time, clock) {
+   // clock(3) is hard to test, but a 1s sleep should cost less than 1ms.
+   clock_t t0 = clock();
+   sleep(1);
+   clock_t t1 = clock();
+   ASSERT_LT(t1 - t0, CLOCKS_PER_SEC / 1000);
+}