diff --git a/docs/api.rst b/docs/api.rst
index 273e31e3..09aa8582 100644
--- a/docs/api.rst
+++ b/docs/api.rst
@@ -603,6 +603,14 @@ Fork Safety
 Utility Functions
 =================
 
+.. currentmodule:: mitogen.core
+.. function:: now
+
+   A reference to :func:`time.time` on Python 2, or :func:`time.monotonic` on
+   Python >3.3. We prefer :func:`time.monotonic` when available to ensure
+   timers are not impacted by system clock changes.
+
+
 .. module:: mitogen.utils
 
 A random assortment of utility functions useful on masters and children.
diff --git a/mitogen/core.py b/mitogen/core.py
index a8de98e2..d6e1739e 100644
--- a/mitogen/core.py
+++ b/mitogen/core.py
@@ -362,6 +362,10 @@ def to_text(o):
     return UnicodeType(o)
 
 
+# Documented in api.rst to work around Sphinx limitation.
+now = getattr(time, 'monotonic', time.time)
+
+
 # Python 2.4
 try:
     any
@@ -636,7 +640,7 @@ def _real_profile_hook(name, func, *args):
         return func(*args)
     finally:
         path = _profile_fmt % {
-            'now': int(1e6 * time.time()),
+            'now': int(1e6 * now()),
             'identity': name,
             'pid': os.getpid(),
             'ext': '%s'
@@ -3482,9 +3486,9 @@ class Broker(object):
         for _, (side, _) in self.poller.readers + self.poller.writers:
             self._call(side.stream, side.stream.on_shutdown)
 
-        deadline = time.time() + self.shutdown_timeout
-        while self.keep_alive() and time.time() < deadline:
-            self._loop_once(max(0, deadline - time.time()))
+        deadline = now() + self.shutdown_timeout
+        while self.keep_alive() and now() < deadline:
+            self._loop_once(max(0, deadline - now()))
 
         if self.keep_alive():
             LOG.error('%r: pending work still existed %d seconds after '
diff --git a/mitogen/master.py b/mitogen/master.py
index 09da775e..b4eb6643 100644
--- a/mitogen/master.py
+++ b/mitogen/master.py
@@ -910,9 +910,9 @@ class ModuleResponder(object):
 
         if self.minify_safe_re.search(source):
             # If the module contains a magic marker, it's safe to minify.
-            t0 = time.time()
+            t0 = mitogen.core.now()
             source = mitogen.minify.minimize_source(source).encode('utf-8')
-            self.minify_secs += time.time() - t0
+            self.minify_secs += mitogen.core.now() - t0
 
         if is_pkg:
             pkg_present = get_child_modules(path)
@@ -1001,11 +1001,11 @@ class ModuleResponder(object):
             LOG.warning('_on_get_module(): dup request for %r from %r',
                         fullname, stream)
 
-        t0 = time.time()
+        t0 = mitogen.core.now()
         try:
             self._send_module_and_related(stream, fullname)
         finally:
-            self.get_module_secs += time.time() - t0
+            self.get_module_secs += mitogen.core.now() - t0
 
     def _send_forward_module(self, stream, context, fullname):
         if stream.protocol.remote_id != context.context_id:
diff --git a/mitogen/parent.py b/mitogen/parent.py
index ec218913..7a94c2b0 100644
--- a/mitogen/parent.py
+++ b/mitogen/parent.py
@@ -633,7 +633,7 @@ class TimerList(object):
     :meth:`expire`. The main user interface to :class:`TimerList` is
     :meth:`schedule`.
     """
-    _now = time.time
+    _now = mitogen.core.now
 
     def __init__(self):
         self._lst = []
@@ -1124,7 +1124,7 @@ class LineLoggingProtocolMixin(object):
 
     def on_line_received(self, line):
         self.logged_partial = None
-        self.logged_lines.append((time.time(), line))
+        self.logged_lines.append((mitogen.core.now(), line))
         self.logged_lines[:] = self.logged_lines[-100:]
         return super(LineLoggingProtocolMixin, self).on_line_received(line)
 
@@ -1134,7 +1134,7 @@ class LineLoggingProtocolMixin(object):
 
     def on_disconnect(self, broker):
         if self.logged_partial:
-            self.logged_lines.append((time.time(), self.logged_partial))
+            self.logged_lines.append((mitogen.core.now(), self.logged_partial))
             self.logged_partial = None
         super(LineLoggingProtocolMixin, self).on_disconnect(broker)
 
@@ -1324,7 +1324,7 @@ class Options(object):
         self.profiling = profiling
         self.unidirectional = unidirectional
         self.max_message_size = max_message_size
-        self.connect_deadline = time.time() + self.connect_timeout
+        self.connect_deadline = mitogen.core.now() + self.connect_timeout
 
 
 class Connection(object):
@@ -1819,7 +1819,7 @@ class CallChain(object):
             socket.gethostname(),
             os.getpid(),
             thread.get_ident(),
-            int(1e6 * time.time()),
+            int(1e6 * mitogen.core.now()),
         )
 
     def __repr__(self):
@@ -2569,7 +2569,7 @@ class Reaper(object):
     def _install_timer(self, delay):
         new = self._timer is None
         self._timer = self.broker.timers.schedule(
-            when=time.time() + delay,
+            when=mitogen.core.now() + delay,
             func=self.reap,
         )
         if new:
diff --git a/mitogen/service.py b/mitogen/service.py
index 6654fb32..8882b30b 100644
--- a/mitogen/service.py
+++ b/mitogen/service.py
@@ -1109,7 +1109,7 @@ class FileService(Service):
             :meth:`fetch`.
         """
         LOG.debug('get_file(): fetching %r from %r', path, context)
-        t0 = time.time()
+        t0 = mitogen.core.now()
         recv = mitogen.core.Receiver(router=context.router)
         metadata = context.call_service(
             service_name=cls.name(),
@@ -1143,5 +1143,6 @@ class FileService(Service):
                       path, metadata['size'], received_bytes)
 
         LOG.debug('target.get_file(): fetched %d bytes of %r from %r in %dms',
-                  metadata['size'], path, context, 1000 * (time.time() - t0))
+                  metadata['size'], path, context,
+                  1000 * (mitogen.core.now() - t0))
         return ok, metadata
diff --git a/tests/bench/fork.py b/tests/bench/fork.py
index b2f2382c..af5cb3a7 100644
--- a/tests/bench/fork.py
+++ b/tests/bench/fork.py
@@ -3,13 +3,13 @@ Measure latency of .fork() setup/teardown.
 """
 
 import mitogen
-import time
+import mitogen.core
 
 @mitogen.main()
 def main(router):
-    t0 = time.time()
+    t0 = mitogen.core.now()
     for x in xrange(200):
-        t = time.time()
+        t = mitogen.core.now()
         ctx = router.fork()
         ctx.shutdown(wait=True)
-    print '++', 1000 * ((time.time() - t0) / (1.0+x))
+    print '++', 1000 * ((mitogen.core.now() - t0) / (1.0+x))
diff --git a/tests/bench/large_messages.py b/tests/bench/large_messages.py
index 24220023..e977e36d 100644
--- a/tests/bench/large_messages.py
+++ b/tests/bench/large_messages.py
@@ -4,7 +4,9 @@
 import subprocess
 import time
 import socket
+
 import mitogen
+import mitogen.core
 
 
 @mitogen.main()
@@ -15,12 +17,12 @@ def main(router):
     s = ' ' * n
     print('bytes in %.2fMiB string...' % (n/1048576.0),)
 
-    t0 = time.time()
+    t0 = mitogen.core.now()
     for x in range(10):
-        tt0 = time.time()
+        tt0 = mitogen.core.now()
         assert n == c.call(len, s)
-        print('took %dms' % (1000 * (time.time() - tt0),))
-    t1 = time.time()
+        print('took %dms' % (1000 * (mitogen.core.now() - tt0),))
+    t1 = mitogen.core.now()
     print('total %dms / %dms avg / %.2fMiB/sec' % (
         1000 * (t1 - t0),
         (1000 * (t1 - t0)) / (x + 1),
diff --git a/tests/bench/latch_roundtrip.py b/tests/bench/latch_roundtrip.py
index 49314fb9..1198aa48 100644
--- a/tests/bench/latch_roundtrip.py
+++ b/tests/bench/latch_roundtrip.py
@@ -6,6 +6,7 @@ import threading
 import time
 
 import mitogen
+import mitogen.core
 import mitogen.utils
 import ansible_mitogen.affinity
 
@@ -33,8 +34,8 @@ t2.start()
 ready.get()
 ready.get()
 
-t0 = time.time()
+t0 = mitogen.core.now()
 l1.put(None)
 t1.join()
 t2.join()
-print('++', int(1e6 * ((time.time() - t0) / (1.0+X))), 'usec')
+print('++', int(1e6 * ((mitogen.core.now() - t0) / (1.0+X))), 'usec')
diff --git a/tests/bench/local.py b/tests/bench/local.py
index 2808d803..aefeb84d 100644
--- a/tests/bench/local.py
+++ b/tests/bench/local.py
@@ -5,6 +5,7 @@ Measure latency of .local() setup.
 import time
 
 import mitogen
+import mitogen.core
 import mitogen.utils
 import ansible_mitogen.affinity
 
@@ -15,10 +16,10 @@ mitogen.utils.setup_gil()
 
 @mitogen.main()
 def main(router):
-    t0=time.time()
+    t0 = mitogen.core.now()
     for x in range(100):
-        t = time.time()
+        t = mitogen.core.now()
         f = router.local()# debug=True)
-        tt = time.time()
+        tt = mitogen.core.now()
         print(x, 1000 * (tt - t))
-    print('%.03f ms' % (1000 * (time.time() - t0) / (1.0 + x)))
+    print('%.03f ms' % (1000 * (mitogen.core.now() - t0) / (1.0 + x)))
diff --git a/tests/bench/megatime.py b/tests/bench/megatime.py
index 6f5f3b5d..40cd9986 100755
--- a/tests/bench/megatime.py
+++ b/tests/bench/megatime.py
@@ -4,12 +4,14 @@ import sys
 import os
 import time
 
+import mitogen.core
+
 
 times = []
 for x in range(5):
-    t0 = time.time()
+    t0 = mitogen.core.now()
     os.spawnvp(os.P_WAIT, sys.argv[1], sys.argv[1:])
-    t = time.time() - t0
+    t = mitogen.core.now() - t0
     times.append(t)
     print('+++', t)
 
diff --git a/tests/bench/roundtrip.py b/tests/bench/roundtrip.py
index 8d86d75b..8f31b1a2 100644
--- a/tests/bench/roundtrip.py
+++ b/tests/bench/roundtrip.py
@@ -5,6 +5,7 @@ Measure latency of local RPC.
 import time
 
 import mitogen
+import mitogen.core
 import mitogen.utils
 import ansible_mitogen.affinity
 
@@ -23,7 +24,7 @@ def do_nothing():
 def main(router):
     f = router.fork()
     f.call(do_nothing)
-    t0 = time.time()
+    t0 = mitogen.core.now()
     for x in xrange(20000):
         f.call(do_nothing)
-    print('++', int(1e6 * ((time.time() - t0) / (1.0+x))), 'usec')
+    print('++', int(1e6 * ((mitogen.core.now() - t0) / (1.0+x))), 'usec')
diff --git a/tests/bench/service.py b/tests/bench/service.py
index 6d866b5c..267ae3f6 100644
--- a/tests/bench/service.py
+++ b/tests/bench/service.py
@@ -4,8 +4,9 @@ Measure latency of local service RPC.
 
 import time
 
-import mitogen.service
 import mitogen
+import mitogen.core
+import mitogen.service
 
 
 class MyService(mitogen.service.Service):
@@ -17,7 +18,7 @@ class MyService(mitogen.service.Service):
 @mitogen.main()
 def main(router):
     f = router.fork()
-    t0 = time.time()
+    t0 = mitogen.core.now()
     for x in range(1000):
         f.call_service(service_name=MyService, method_name='ping')
-    print('++', int(1e6 * ((time.time() - t0) / (1.0+x))), 'usec')
+    print('++', int(1e6 * ((mitogen.core.now() - t0) / (1.0+x))), 'usec')
diff --git a/tests/bench/ssh-roundtrip.py b/tests/bench/ssh-roundtrip.py
index 8745505d..06c596c0 100644
--- a/tests/bench/ssh-roundtrip.py
+++ b/tests/bench/ssh-roundtrip.py
@@ -6,6 +6,7 @@ import sys
 import time
 
 import mitogen
+import mitogen.core
 import mitogen.utils
 import ansible_mitogen.affinity
 
@@ -24,12 +25,12 @@ def do_nothing():
 def main(router):
     f = router.ssh(hostname=sys.argv[1])
     f.call(do_nothing)
-    t0 = time.time()
-    end = time.time() + 5.0
+    t0 = mitogen.core.now()
+    end = mitogen.core.now() + 5.0
     i = 0
-    while time.time() < end:
+    while mitogen.core.now() < end:
         f.call(do_nothing)
         i += 1
-        t1 = time.time()
+        t1 = mitogen.core.now()
 
     print('++', float(1e3 * (t1 - t0) / (1.0+i)), 'ms')
diff --git a/tests/bench/throughput.py b/tests/bench/throughput.py
index 42604826..acb51afa 100644
--- a/tests/bench/throughput.py
+++ b/tests/bench/throughput.py
@@ -8,6 +8,7 @@ import tempfile
 import time
 
 import mitogen
+import mitogen.core
 import mitogen.service
 import ansible_mitogen.affinity
 
@@ -35,9 +36,9 @@ def run_test(router, fp, s, context):
     size = fp.tell()
     print('Testing %s...' % (s,))
     context.call(prepare)
-    t0 = time.time()
+    t0 = mitogen.core.now()
     context.call(transfer, router.myself(), fp.name)
-    t1 = time.time()
+    t1 = mitogen.core.now()
     print('%s took %.2f ms to transfer %.2f MiB, %.2f MiB/s' % (
         s, 1000 * (t1 - t0), size / 1048576.0,
         (size / (t1 - t0) / 1048576.0),
diff --git a/tests/create_child_test.py b/tests/create_child_test.py
index 1c2f526a..26f10d57 100644
--- a/tests/create_child_test.py
+++ b/tests/create_child_test.py
@@ -9,6 +9,7 @@ import tempfile
 import mock
 import unittest2
 
+import mitogen.core
 import mitogen.parent
 from mitogen.core import b
 
@@ -188,7 +189,6 @@ class TtyCreateChildTest(testlib.TestCase):
             proc = self.func([
                 'bash', '-c', 'exec 2>%s; echo hi > /dev/tty' % (tf.name,)
             ])
-            deadline = time.time() + 5.0
             mitogen.core.set_block(proc.stdin.fileno())
             # read(3) below due to https://bugs.python.org/issue37696
             self.assertEquals(mitogen.core.b('hi\n'), proc.stdin.read(3))
@@ -271,7 +271,6 @@ class TtyCreateChildTest(testlib.TestCase):
             proc = self.func([
                 'bash', '-c', 'exec 2>%s; echo hi > /dev/tty' % (tf.name,)
             ])
-            deadline = time.time() + 5.0
             self.assertEquals(mitogen.core.b('hi\n'), wait_read(proc.stdout, 3))
             waited_pid, status = os.waitpid(proc.pid, 0)
             self.assertEquals(proc.pid, waited_pid)
diff --git a/tests/parent_test.py b/tests/parent_test.py
index b314d472..d6efe998 100644
--- a/tests/parent_test.py
+++ b/tests/parent_test.py
@@ -12,6 +12,7 @@ import unittest2
 import testlib
 from testlib import Popen__terminate
 
+import mitogen.core
 import mitogen.parent
 
 try:
@@ -21,8 +22,8 @@ except NameError:
 
 
 def wait_for_child(pid, timeout=1.0):
-    deadline = time.time() + timeout
-    while timeout < time.time():
+    deadline = mitogen.core.now() + timeout
+    while timeout < mitogen.core.now():
         try:
             target_pid, status = os.waitpid(pid, os.WNOHANG)
             if target_pid == pid:
diff --git a/tests/poller_test.py b/tests/poller_test.py
index b05a9b94..3ed59ae3 100644
--- a/tests/poller_test.py
+++ b/tests/poller_test.py
@@ -164,14 +164,14 @@ class CloseMixin(PollerMixin):
 
 class PollMixin(PollerMixin):
     def test_empty_zero_timeout(self):
-        t0 = time.time()
+        t0 = mitogen.core.now()
         self.assertEquals([], list(self.p.poll(0)))
-        self.assertTrue((time.time() - t0) < .1)  # vaguely reasonable
+        self.assertTrue((mitogen.core.now() - t0) < .1)  # vaguely reasonable
 
     def test_empty_small_timeout(self):
-        t0 = time.time()
+        t0 = mitogen.core.now()
         self.assertEquals([], list(self.p.poll(.2)))
-        self.assertTrue((time.time() - t0) >= .2)
+        self.assertTrue((mitogen.core.now() - t0) >= .2)
 
 
 class ReadableMixin(PollerMixin, SockMixin):
diff --git a/tests/testlib.py b/tests/testlib.py
index 255fba88..73d3438d 100644
--- a/tests/testlib.py
+++ b/tests/testlib.py
@@ -107,11 +107,11 @@ def wait_for_port(
     If a regex pattern is supplied try to find it in the initial data.
     Return None on success, or raise on error.
     """
-    start = time.time()
+    start = mitogen.core.now()
     end = start + overall_timeout
     addr = (host, port)
 
-    while time.time() < end:
+    while mitogen.core.now() < end:
         sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
         sock.settimeout(connect_timeout)
         try:
@@ -130,7 +130,7 @@ def wait_for_port(
         sock.settimeout(receive_timeout)
         data = mitogen.core.b('')
         found = False
-        while time.time() < end:
+        while mitogen.core.now() < end:
             try:
                 resp = sock.recv(1024)
             except socket.timeout:
diff --git a/tests/timer_test.py b/tests/timer_test.py
index ff3e022f..749405a4 100644
--- a/tests/timer_test.py
+++ b/tests/timer_test.py
@@ -162,7 +162,7 @@ def do_timer_test_econtext(econtext):
 
 
 def do_timer_test(broker):
-    now = time.time()
+    now = mitogen.core.now()
     latch = mitogen.core.Latch()
     broker.defer(lambda:
         broker.timers.schedule(
@@ -172,7 +172,7 @@ def do_timer_test(broker):
     )
 
     assert 'hi' == latch.get()
-    assert time.time() > (now + 0.250)
+    assert mitogen.core.now() > (now + 0.250)
 
 
 class BrokerTimerTest(testlib.TestCase):
diff --git a/tests/unix_test.py b/tests/unix_test.py
index ba1ba152..cf3e595f 100644
--- a/tests/unix_test.py
+++ b/tests/unix_test.py
@@ -86,12 +86,12 @@ class ClientTest(testlib.TestCase):
 
     def _try_connect(self, path):
         # give server a chance to setup listener
-        timeout = time.time() + 30.0
+        timeout = mitogen.core.now() + 30.0
         while True:
             try:
                 return mitogen.unix.connect(path)
             except mitogen.unix.ConnectError:
-                if time.time() > timeout:
+                if mitogen.core.now() > timeout:
                     raise
                 time.sleep(0.1)