diff --git a/humanfriendly/__init__.py b/humanfriendly/__init__.py
index 4c0a333..e2abc04 100644
--- a/humanfriendly/__init__.py
+++ b/humanfriendly/__init__.py
@@ -656,9 +656,9 @@ def __init__(self, start_time=None, resumable=False):
         if resumable:
             self.monotonic = True
             self.resumable = True
-            self.start_time = 0.0
+            self.start_time = None
             self.total_time = 0.0
-        elif start_time:
+        elif start_time is not None:
             self.monotonic = False
             self.resumable = False
             self.start_time = start_time
@@ -687,9 +687,9 @@ def __exit__(self, exc_type=None, exc_value=None, traceback=None):
         """
         if not self.resumable:
             raise ValueError("Timer is not resumable!")
-        if self.start_time:
+        if self.start_time is not None:
             self.total_time += monotonic() - self.start_time
-            self.start_time = 0.0
+            self.start_time = None
 
     def sleep(self, seconds):
         """
@@ -725,7 +725,7 @@ def elapsed_time(self):
         elapsed_time = 0
         if self.resumable:
             elapsed_time += self.total_time
-        if self.start_time:
+        if self.start_time is not None:
             current_time = monotonic() if self.monotonic else time.time()
             elapsed_time += current_time - self.start_time
         return elapsed_time
diff --git a/humanfriendly/tests.py b/humanfriendly/tests.py
index 72dad99..ea5d59a 100644
--- a/humanfriendly/tests.py
+++ b/humanfriendly/tests.py
@@ -744,6 +744,47 @@ def test_split(self):
         self.assertEqual(split('foo, bar, baz'), ['foo', 'bar', 'baz'])
         self.assertEqual(split('foo,bar,baz'), ['foo', 'bar', 'baz'])
 
+    def test_timer_epoch_start(self):
+        """An explicit epoch start uses the wall clock, including zero."""
+        with PatchedAttribute(time, 'time', lambda: 12.5):
+            for start in (0, 0.0, -2.0, 2.0):
+                timer = Timer(start_time=start)
+                self.assertFalse(timer.monotonic)
+                self.assertEqual(timer.elapsed_time, 12.5 - start)
+
+    def test_timer_zero_monotonic_start(self):
+        """A zero monotonic reading remains an active timer."""
+        with PatchedItem(Timer.__init__.__globals__, 'monotonic', lambda: 0.0):
+            automatic = Timer()
+            resumable = Timer(resumable=True)
+            resumable.__enter__()
+        with PatchedItem(Timer.__init__.__globals__, 'monotonic', lambda: 2.5):
+            self.assertEqual(automatic.elapsed_time, 2.5)
+            self.assertEqual(resumable.elapsed_time, 2.5)
+            resumable.__exit__(None, None, None)
+            self.assertEqual(resumable.elapsed_time, 2.5)
+        with PatchedItem(Timer.__init__.__globals__, 'monotonic', lambda: 4.0):
+            self.assertEqual(resumable.elapsed_time, 2.5)
+
+    def test_timer_inactive_and_resumed(self):
+        """Paused timers stay fixed and resume accumulating elapsed time."""
+        with PatchedItem(Timer.__init__.__globals__, 'monotonic', lambda: 5.0):
+            timer = Timer(resumable=True)
+            self.assertEqual(timer.elapsed_time, 0)
+            timer.__exit__(None, None, None)
+            self.assertEqual(timer.elapsed_time, 0)
+            self.assertIs(timer.__enter__(), timer)
+        with PatchedItem(Timer.__init__.__globals__, 'monotonic', lambda: 7.0):
+            timer.__exit__(None, None, None)
+            self.assertEqual(timer.elapsed_time, 2.0)
+            timer.__exit__(None, None, None)
+            self.assertEqual(timer.elapsed_time, 2.0)
+            timer.__enter__()
+        with PatchedItem(Timer.__init__.__globals__, 'monotonic', lambda: 10.0):
+            self.assertEqual(timer.elapsed_time, 5.0)
+            timer.__exit__(None, None, None)
+            self.assertEqual(timer.elapsed_time, 5.0)
+
     def test_timer(self):
         """Test :func:`humanfriendly.Timer`."""
         for seconds, text in ((1, '1 second'),