diff --git a/Lib/pprint.py b/Lib/pprint.py
index bcf2eedebe6..f2a117864e5 100644
--- a/Lib/pprint.py
+++ b/Lib/pprint.py
@@ -568,11 +568,11 @@ def _perfcheck(object=None):
     if object is None:
         object = [("string", (1, 2), [3, 4], {5: 6, 7: 8})] * 100000
     p = PrettyPrinter()
-    t1 = time.time()
+    t1 = time.perf_counter()
     _safe_repr(object, {}, None, 0)
-    t2 = time.time()
+    t2 = time.perf_counter()
     p.pformat(object)
-    t3 = time.time()
+    t3 = time.perf_counter()
     print("_safe_repr:", t2 - t1)
     print("pformat:", t3 - t2)
 
diff --git a/Lib/random.py b/Lib/random.py
index 03c058a39d6..9c2904cfd2f 100644
--- a/Lib/random.py
+++ b/Lib/random.py
@@ -740,14 +740,14 @@ def _test_generator(n, func, args):
     sqsum = 0.0
     smallest = 1e10
     largest = -1e10
-    t0 = time.time()
+    t0 = time.perf_counter()
     for i in range(n):
         x = func(*args)
         total += x
         sqsum = sqsum + x*x
         smallest = min(x, smallest)
         largest = max(x, largest)
-    t1 = time.time()
+    t1 = time.perf_counter()
     print(round(t1-t0, 3), 'sec,', end=' ')
     avg = total/n
     stddev = _sqrt(sqsum/n - avg*avg)
diff --git a/Lib/test/libregrtest/runtest.py b/Lib/test/libregrtest/runtest.py
index 466b522db9e..dc2abf237bc 100644
--- a/Lib/test/libregrtest/runtest.py
+++ b/Lib/test/libregrtest/runtest.py
@@ -162,7 +162,7 @@ def runtest_inner(ns, test, display_failure=True):
         abstest = get_abs_module(ns, test)
         clear_caches()
         with saved_test_environment(test, ns.verbose, ns.quiet, pgo=ns.pgo) as environment:
-            start_time = time.time()
+            start_time = time.perf_counter()
             the_module = importlib.import_module(abstest)
             # If the test has a test_main, that will run the appropriate
             # tests.  If not, use normal unittest test loading.
@@ -180,7 +180,7 @@ def runtest_inner(ns, test, display_failure=True):
                 refleak = dash_R(the_module, test, test_runner, ns.huntrleaks)
             else:
                 test_runner()
-            test_time = time.time() - start_time
+            test_time = time.perf_counter() - start_time
         post_test_cleanup()
     except support.ResourceDenied as msg:
         if not ns.quiet and not ns.pgo:
diff --git a/Lib/test/time_hashlib.py b/Lib/test/time_hashlib.py
index 2585ecb7820..55ebac62912 100644
--- a/Lib/test/time_hashlib.py
+++ b/Lib/test/time_hashlib.py
@@ -14,26 +14,26 @@ def test_scaled_msg(scale, name):
     longStr = b'Z'*scale
 
     localCF = creatorFunc
-    start = time.time()
+    start = time.perf_counter()
     for f in range(iterations):
         x = localCF(longStr).digest()
-    end = time.time()
+    end = time.perf_counter()
 
     print(('%2.2f' % (end-start)), "seconds", iterations, "x", len(longStr), "bytes", name)
 
 def test_create():
-    start = time.time()
+    start = time.perf_counter()
     for f in range(20000):
         d = creatorFunc()
-    end = time.time()
+    end = time.perf_counter()
 
     print(('%2.2f' % (end-start)), "seconds", '[20000 creations]')
 
 def test_zero():
-    start = time.time()
+    start = time.perf_counter()
     for f in range(20000):
         x = creatorFunc().digest()
-    end = time.time()
+    end = time.perf_counter()
 
     print(('%2.2f' % (end-start)), "seconds", '[20000 "" digests]')
 
diff --git a/Lib/unittest/runner.py b/Lib/unittest/runner.py
index 2c5ea4ab079..45e7e4c0458 100644
--- a/Lib/unittest/runner.py
+++ b/Lib/unittest/runner.py
@@ -168,7 +168,7 @@ class TextTestRunner(object):
                     warnings.filterwarnings('module',
                             category=DeprecationWarning,
                             message=r'Please use assert\w+ instead.')
-            startTime = time.time()
+            startTime = time.perf_counter()
             startTestRun = getattr(result, 'startTestRun', None)
             if startTestRun is not None:
                 startTestRun()
@@ -178,7 +178,7 @@ class TextTestRunner(object):
                 stopTestRun = getattr(result, 'stopTestRun', None)
                 if stopTestRun is not None:
                     stopTestRun()
-            stopTime = time.time()
+            stopTime = time.perf_counter()
         timeTaken = stopTime - startTime
         result.printErrors()
         if hasattr(result, 'separator2'):
diff --git a/Misc/NEWS.d/next/Library/2018-12-16-23-28-49.bpo-35513.pn-Zh3.rst b/Misc/NEWS.d/next/Library/2018-12-16-23-28-49.bpo-35513.pn-Zh3.rst
new file mode 100644
index 00000000000..f1436a718de
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2018-12-16-23-28-49.bpo-35513.pn-Zh3.rst
@@ -0,0 +1,4 @@
+:class:`~unittest.runner.TextTestRunner` of :mod:`unittest.runner` now uses
+:func:`time.perf_counter` rather than :func:`time.time` to measure the
+execution time of a test: :func:`time.time` can go backwards, whereas
+:func:`time.perf_counter` is monotonic.