diff --git a/backoff/_async.py b/backoff/_async.py
index 82fd477..e3fac73 100644
--- a/backoff/_async.py
+++ b/backoff/_async.py
@@ -2,6 +2,7 @@
 import datetime
 import functools
 import asyncio
+import time
 from datetime import timedelta
 
 from backoff._common import (_init_wait_gen, _maybe_call, _next_wait)
@@ -60,11 +61,11 @@ async def retry(*args, **kwargs):
         max_time_value = _maybe_call(max_time)
 
         tries = 0
-        start = datetime.datetime.now()
+        start = time.monotonic()
         wait = _init_wait_gen(wait_gen, wait_gen_kwargs)
         while True:
             tries += 1
-            elapsed = timedelta.total_seconds(datetime.datetime.now() - start)
+            elapsed = time.monotonic() - start
             details = {
                 "target": target,
                 "args": args,
@@ -134,11 +135,11 @@ async def retry(*args, **kwargs):
         max_time_value = _maybe_call(max_time)
 
         tries = 0
-        start = datetime.datetime.now()
+        start = time.monotonic()
         wait = _init_wait_gen(wait_gen, wait_gen_kwargs)
         while True:
             tries += 1
-            elapsed = timedelta.total_seconds(datetime.datetime.now() - start)
+            elapsed = time.monotonic() - start
             details = {
                 "target": target,
                 "args": args,
diff --git a/backoff/_sync.py b/backoff/_sync.py
index 4371e16..0ed6860 100644
--- a/backoff/_sync.py
+++ b/backoff/_sync.py
@@ -1,5 +1,4 @@
 # coding:utf-8
-import datetime
 import functools
 import time
 from datetime import timedelta
@@ -32,11 +31,11 @@ def retry(*args, **kwargs):
         max_time_value = _maybe_call(max_time)
 
         tries = 0
-        start = datetime.datetime.now()
+        start = time.monotonic()
         wait = _init_wait_gen(wait_gen, wait_gen_kwargs)
         while True:
             tries += 1
-            elapsed = timedelta.total_seconds(datetime.datetime.now() - start)
+            elapsed = time.monotonic() - start
             details = {
                 "target": target,
                 "args": args,
@@ -88,11 +87,11 @@ def retry(*args, **kwargs):
         max_time_value = _maybe_call(max_time)
 
         tries = 0
-        start = datetime.datetime.now()
+        start = time.monotonic()
         wait = _init_wait_gen(wait_gen, wait_gen_kwargs)
         while True:
             tries += 1
-            elapsed = timedelta.total_seconds(datetime.datetime.now() - start)
+            elapsed = time.monotonic() - start
             details = {
                 "target": target,
                 "args": args,
diff --git a/tests/test_backoff.py b/tests/test_backoff.py
index cd33b63..9556cd0 100644
--- a/tests/test_backoff.py
+++ b/tests/test_backoff.py
@@ -46,19 +46,19 @@ def return_true(log, n):
 
 def test_on_predicate_max_time(monkeypatch):
     nows = [
-        datetime.datetime(2018, 1, 1, 12, 0, 10, 5),
-        datetime.datetime(2018, 1, 1, 12, 0, 9, 0),
-        datetime.datetime(2018, 1, 1, 12, 0, 1, 0),
-        datetime.datetime(2018, 1, 1, 12, 0, 0, 0),
+        10.000005,
+        9,
+        1,
+        0
     ]
 
-    class Datetime:
+    class Time:
         @staticmethod
-        def now():
+        def monotonic():
             return nows.pop()
 
     monkeypatch.setattr('time.sleep', lambda x: None)
-    monkeypatch.setattr('datetime.datetime', Datetime)
+    monkeypatch.setattr('time.monotonic', Time.monotonic)
 
     def giveup(details):
         assert details['tries'] == 3
@@ -79,19 +79,19 @@ def return_true(log, n):
 
 def test_on_predicate_max_time_callable(monkeypatch):
     nows = [
-        datetime.datetime(2018, 1, 1, 12, 0, 10, 5),
-        datetime.datetime(2018, 1, 1, 12, 0, 9, 0),
-        datetime.datetime(2018, 1, 1, 12, 0, 1, 0),
-        datetime.datetime(2018, 1, 1, 12, 0, 0, 0),
+        10.000005,
+        9,
+        1,
+        0
     ]
 
-    class Datetime:
+    class Time:
         @staticmethod
-        def now():
+        def monotonic():
             return nows.pop()
 
     monkeypatch.setattr('time.sleep', lambda x: None)
-    monkeypatch.setattr('datetime.datetime', Datetime)
+    monkeypatch.setattr('time.monotonic', Time.monotonic)
 
     def giveup(details):
         assert details['tries'] == 3