diff --git a/common/mclock/mclock.go b/common/mclock/mclock.go
index 0c941082f3..d0e0cd78be 100644
--- a/common/mclock/mclock.go
+++ b/common/mclock/mclock.go
@@ -36,47 +36,39 @@ func (t AbsTime) Add(d time.Duration) AbsTime {
 	return t + AbsTime(d)
 }
 
-// Clock interface makes it possible to replace the monotonic system clock with
+// The Clock interface makes it possible to replace the monotonic system clock with
 // a simulated clock.
 type Clock interface {
 	Now() AbsTime
 	Sleep(time.Duration)
 	After(time.Duration) <-chan time.Time
-	AfterFunc(d time.Duration, f func()) Event
+	AfterFunc(d time.Duration, f func()) Timer
 }
 
-// Event represents a cancellable event returned by AfterFunc
-type Event interface {
-	Cancel() bool
+// Timer represents a cancellable event returned by AfterFunc
+type Timer interface {
+	Stop() bool
 }
 
 // System implements Clock using the system clock.
 type System struct{}
 
-// Now implements Clock.
+// Now returns the current monotonic time.
 func (System) Now() AbsTime {
 	return AbsTime(monotime.Now())
 }
 
-// Sleep implements Clock.
+// Sleep blocks for the given duration.
 func (System) Sleep(d time.Duration) {
 	time.Sleep(d)
 }
 
-// After implements Clock.
+// After returns a channel which receives the current time after d has elapsed.
 func (System) After(d time.Duration) <-chan time.Time {
 	return time.After(d)
 }
 
-// AfterFunc implements Clock.
-func (System) AfterFunc(d time.Duration, f func()) Event {
-	return (*SystemEvent)(time.AfterFunc(d, f))
-}
-
-// SystemEvent implements Event using time.Timer.
-type SystemEvent time.Timer
-
-// Cancel implements Event.
-func (e *SystemEvent) Cancel() bool {
-	return (*time.Timer)(e).Stop()
+// AfterFunc runs f on a new goroutine after the duration has elapsed.
+func (System) AfterFunc(d time.Duration, f func()) Timer {
+	return time.AfterFunc(d, f)
 }
diff --git a/common/mclock/simclock.go b/common/mclock/simclock.go
index af0f71c430..4d351252ff 100644
--- a/common/mclock/simclock.go
+++ b/common/mclock/simclock.go
@@ -32,22 +32,17 @@ import (
 // the timeout using a channel or semaphore.
 type Simulated struct {
 	now       AbsTime
-	scheduled []event
+	scheduled []*simTimer
 	mu        sync.RWMutex
 	cond      *sync.Cond
 	lastId    uint64
 }
 
-type event struct {
+// simTimer implements Timer on the virtual clock.
+type simTimer struct {
 	do func()
 	at AbsTime
 	id uint64
-}
-
-// SimulatedEvent implements Event for a virtual clock.
-type SimulatedEvent struct {
-	at AbsTime
-	id uint64
 	s  *Simulated
 }
 
@@ -75,6 +70,7 @@ func (s *Simulated) Run(d time.Duration) {
 	}
 }
 
+// ActiveTimers returns the number of timers that haven't fired.
 func (s *Simulated) ActiveTimers() int {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
@@ -82,6 +78,7 @@ func (s *Simulated) ActiveTimers() int {
 	return len(s.scheduled)
 }
 
+// WaitForTimers waits until the clock has at least n scheduled timers.
 func (s *Simulated) WaitForTimers(n int) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
@@ -92,7 +89,7 @@ func (s *Simulated) WaitForTimers(n int) {
 	}
 }
 
-// Now implements Clock.
+// Now returns the current virtual time.
 func (s *Simulated) Now() AbsTime {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
@@ -100,12 +97,13 @@ func (s *Simulated) Now() AbsTime {
 	return s.now
 }
 
-// Sleep implements Clock.
+// Sleep blocks until the clock has advanced by d.
 func (s *Simulated) Sleep(d time.Duration) {
 	<-s.After(d)
 }
 
-// After implements Clock.
+// After returns a channel which receives the current time after the clock
+// has advanced by d.
 func (s *Simulated) After(d time.Duration) <-chan time.Time {
 	after := make(chan time.Time, 1)
 	s.AfterFunc(d, func() {
@@ -114,8 +112,9 @@ func (s *Simulated) After(d time.Duration) <-chan time.Time {
 	return after
 }
 
-// AfterFunc implements Clock.
-func (s *Simulated) AfterFunc(d time.Duration, do func()) Event {
+// AfterFunc runs fn after the clock has advanced by d. Unlike with the system
+// clock, fn runs on the goroutine that calls Run.
+func (s *Simulated) AfterFunc(d time.Duration, fn func()) Timer {
 	s.mu.Lock()
 	defer s.mu.Unlock()
 	s.init()
@@ -133,44 +132,31 @@ func (s *Simulated) AfterFunc(d time.Duration, do func()) Event {
 			l = m + 1
 		}
 	}
-	s.scheduled = append(s.scheduled, event{})
+	ev := &simTimer{do: fn, at: at, s: s}
+	s.scheduled = append(s.scheduled, nil)
 	copy(s.scheduled[l+1:], s.scheduled[l:ll])
-	e := event{do: do, at: at, id: id}
-	s.scheduled[l] = e
+	s.scheduled[l] = ev
 	s.cond.Broadcast()
-	return &SimulatedEvent{at: at, id: id, s: s}
-}
-
-func (s *Simulated) init() {
-	if s.cond == nil {
-		s.cond = sync.NewCond(&s.mu)
-	}
+	return ev
 }
 
-// Cancel implements Event.
-func (e *SimulatedEvent) Cancel() bool {
-	s := e.s
+func (ev *simTimer) Stop() bool {
+	s := ev.s
 	s.mu.Lock()
 	defer s.mu.Unlock()
 
-	l, h := 0, len(s.scheduled)
-	ll := h
-	for l != h {
-		m := (l + h) / 2
-		if e.id == s.scheduled[m].id {
-			l = m
-			break
-		}
-		if (e.at < s.scheduled[m].at) || ((e.at == s.scheduled[m].at) && (e.id < s.scheduled[m].id)) {
-			h = m
-		} else {
-			l = m + 1
+	for i := 0; i < len(s.scheduled); i++ {
+		if s.scheduled[i] == ev {
+			s.scheduled = append(s.scheduled[:i], s.scheduled[i+1:]...)
+			s.cond.Broadcast()
+			return true
 		}
 	}
-	if l >= ll || s.scheduled[l].id != e.id {
-		return false
+	return false
+}
+
+func (s *Simulated) init() {
+	if s.cond == nil {
+		s.cond = sync.NewCond(&s.mu)
 	}
-	copy(s.scheduled[l:ll-1], s.scheduled[l+1:])
-	s.scheduled = s.scheduled[:ll-1]
-	return true
 }
diff --git a/common/mclock/simclock_test.go b/common/mclock/simclock_test.go
new file mode 100644
index 0000000000..09e4391c1c
--- /dev/null
+++ b/common/mclock/simclock_test.go
@@ -0,0 +1,115 @@
+// Copyright 2018 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package mclock
+
+import (
+	"testing"
+	"time"
+)
+
+var _ Clock = System{}
+var _ Clock = new(Simulated)
+
+func TestSimulatedAfter(t *testing.T) {
+	const timeout = 30 * time.Minute
+	const adv = time.Minute
+
+	var (
+		c   Simulated
+		end = c.Now().Add(timeout)
+		ch  = c.After(timeout)
+	)
+	for c.Now() < end.Add(-adv) {
+		c.Run(adv)
+		select {
+		case <-ch:
+			t.Fatal("Timer fired early")
+		default:
+		}
+	}
+
+	c.Run(adv)
+	select {
+	case stamp := <-ch:
+		want := time.Time{}.Add(timeout)
+		if !stamp.Equal(want) {
+			t.Errorf("Wrong time sent on timer channel: got %v, want %v", stamp, want)
+		}
+	default:
+		t.Fatal("Timer didn't fire")
+	}
+}
+
+func TestSimulatedAfterFunc(t *testing.T) {
+	var c Simulated
+
+	called1 := false
+	timer1 := c.AfterFunc(100*time.Millisecond, func() { called1 = true })
+	if c.ActiveTimers() != 1 {
+		t.Fatalf("%d active timers, want one", c.ActiveTimers())
+	}
+	if fired := timer1.Stop(); !fired {
+		t.Fatal("Stop returned false even though timer didn't fire")
+	}
+	if c.ActiveTimers() != 0 {
+		t.Fatalf("%d active timers, want zero", c.ActiveTimers())
+	}
+	if called1 {
+		t.Fatal("timer 1 called")
+	}
+	if fired := timer1.Stop(); fired {
+		t.Fatal("Stop returned true after timer was already stopped")
+	}
+
+	called2 := false
+	timer2 := c.AfterFunc(100*time.Millisecond, func() { called2 = true })
+	c.Run(50 * time.Millisecond)
+	if called2 {
+		t.Fatal("timer 2 called")
+	}
+	c.Run(51 * time.Millisecond)
+	if !called2 {
+		t.Fatal("timer 2 not called")
+	}
+	if fired := timer2.Stop(); fired {
+		t.Fatal("Stop returned true after timer has fired")
+	}
+}
+
+func TestSimulatedSleep(t *testing.T) {
+	var (
+		c       Simulated
+		timeout = 1 * time.Hour
+		done    = make(chan AbsTime)
+	)
+	go func() {
+		c.Sleep(timeout)
+		done <- c.Now()
+	}()
+
+	c.WaitForTimers(1)
+	c.Run(2 * timeout)
+	select {
+	case stamp := <-done:
+		want := AbsTime(2 * timeout)
+		if stamp != want {
+			t.Errorf("Wrong time after sleep: got %v, want %v", stamp, want)
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("Sleep didn't return in time")
+	}
+}
diff --git a/les/balance.go b/les/balance.go
index 4f08a304eb..a36a997cf3 100644
--- a/les/balance.go
+++ b/les/balance.go
@@ -42,7 +42,7 @@ type balanceTracker struct {
 	negTimeFactor, negRequestFactor  float64
 	sumReqCost                       uint64
 	lastUpdate, nextUpdate, initTime mclock.AbsTime
-	updateEvent                      mclock.Event
+	updateEvent                      mclock.Timer
 	// since only a limited and fixed number of callbacks are needed, they are
 	// stored in a fixed size array ordered by priority threshold.
 	callbacks [balanceCallbackCount]balanceCallback
@@ -86,7 +86,7 @@ func (bt *balanceTracker) stop(now mclock.AbsTime) {
 	bt.timeFactor = 0
 	bt.requestFactor = 0
 	if bt.updateEvent != nil {
-		bt.updateEvent.Cancel()
+		bt.updateEvent.Stop()
 		bt.updateEvent = nil
 	}
 }
@@ -235,7 +235,7 @@ func (bt *balanceTracker) checkCallbacks(now mclock.AbsTime) {
 
 // updateAfter schedules a balance update and callback check in the future
 func (bt *balanceTracker) updateAfter(dt time.Duration) {
-	if bt.updateEvent == nil || bt.updateEvent.Cancel() {
+	if bt.updateEvent == nil || bt.updateEvent.Stop() {
 		if dt == 0 {
 			bt.updateEvent = nil
 		} else {