diff --git a/cmd/vail/book.go b/cmd/vail/book.go
index 6f8bafa..f8573b9 100644
--- a/cmd/vail/book.go
+++ b/cmd/vail/book.go
@@ -5,15 +5,20 @@ import (
 	"log"
 )
 
+// Book maps names to repeaters
+//
+// It ensures that names map 1-1 to repeaters.
 type Book struct {
-	entries map[string]*Repeater
-	events  chan bookEvent
+	entries      map[string]*Repeater
+	events       chan bookEvent
+	makeRepeater func() *Repeater
 }
 
 func NewBook() Book {
 	return Book{
-		entries: make(map[string]*Repeater),
-		events:  make(chan bookEvent, 5),
+		entries:      make(map[string]*Repeater),
+		events:       make(chan bookEvent, 5),
+		makeRepeater: NewRepeater,
 	}
 }
 
@@ -32,6 +37,7 @@ type bookEvent struct {
 	m         Message
 }
 
+// Join adds a writer to a named repeater
 func (b Book) Join(name string, w io.Writer) {
 	b.events <- bookEvent{
 		eventType: joinEvent,
@@ -40,6 +46,7 @@ func (b Book) Join(name string, w io.Writer) {
 	}
 }
 
+// Part removes a writer from a named repeater
 func (b Book) Part(name string, w io.Writer) {
 	b.events <- bookEvent{
 		eventType: partEvent,
@@ -48,6 +55,7 @@ func (b Book) Part(name string, w io.Writer) {
 	}
 }
 
+// Send transmits a message to the named repeater
 func (b Book) Send(name string, m Message) {
 	b.events <- bookEvent{
 		eventType: sendEvent,
@@ -56,6 +64,7 @@ func (b Book) Send(name string, m Message) {
 	}
 }
 
+// Run is the endless run loop
 func (b Book) Run() {
 	for {
 		b.loop()
@@ -69,7 +78,7 @@ func (b Book) loop() {
 	switch event.eventType {
 	case joinEvent:
 		if !ok {
-			repeater = NewRepeater()
+			repeater = b.makeRepeater()
 			b.entries[event.name] = repeater
 		}
 		repeater.Join(event.w)
diff --git a/cmd/vail/book_test.go b/cmd/vail/book_test.go
index fd59482..aa66412 100644
--- a/cmd/vail/book_test.go
+++ b/cmd/vail/book_test.go
@@ -1,69 +1,57 @@
 package main
 
 import (
-	"bytes"
 	"testing"
 )
 
 func TestBook(t *testing.T) {
-	b := NewBook()
-	m := TestMessage{Message{1, 2, []uint8{3, 4}}}
+	b := Book{
+		entries:      make(map[string]*Repeater),
+		events:       make(chan bookEvent, 5),
+		makeRepeater: NewTestingRepeater,
+	}
 
-	buf1 := bytes.NewBufferString("buf1")
-	buf1Expect := bytes.NewBufferString("buf1")
-	b.Join("moo", buf1)
-	m.Clients = 1
+	c1 := NewTestingClient(t)
+	b.Join("moo", c1)
 	b.loop()
 	if len(b.entries) != 1 {
 		t.Error("Wrong number of entries")
 	}
+	c1.Expect(1)
 
 	// Send to an empty channel
-	b.Send("merf", m.Message)
+	m := Message{0, 0, []uint8{22, 33}}
+	b.Send("merf", m)
 	b.loop()
-	if buf1.String() != buf1Expect.String() {
+	if c1.Len() > 0 {
 		t.Error("Sending to empty channel sent to non-empty channel")
 	}
 
 	// Send to a non-empty channel!
-	b.Send("moo", m.Message)
+	b.Send("moo", m)
 	b.loop()
-	buf1Expect.Write(m.bytes())
-	if buf1.String() != buf1Expect.String() {
-		t.Error("Sending didn't work")
-	}
+	c1.Expect(1, 22, 33)
 
 	// Join another client
-	buf2 := bytes.NewBufferString("buf2")
-	buf2Expect := bytes.NewBufferString("buf2")
-	b.Join("moo", buf2)
-	m.Clients = 2
+	c2 := NewTestingClient(t)
+	b.Join("moo", c2)
 	b.loop()
+	c1.Expect(2)
+	c2.Expect(2)
 
 	// Send to both
-	b.Send("moo", m.Message)
+	m.Duration = append(m.Duration, 44)
+	b.Send("moo", m)
 	b.loop()
-	buf1Expect.Write(m.bytes())
-	buf2Expect.Write(m.bytes())
-	if buf1.String() != buf1Expect.String() {
-		t.Error("Send to 2-member channel busted", buf1)
-	}
-	if buf2.String() != buf2Expect.String() {
-		t.Error("Send to 2-member channel busted", buf2)
-	}
+	c1.Expect(2, 22, 33, 44)
+	c2.Expect(2, 22, 33, 44)
 
 	// Part a client
-	b.Part("moo", buf1)
+	b.Part("moo", c1)
 	b.loop()
-	m.Clients = 1
+	c2.Expect(1)
 
-	b.Send("moo", m.Message)
+	b.Send("moo", m)
 	b.loop()
-	buf2Expect.Write(m.bytes())
-	if buf1.String() != buf1Expect.String() {
-		t.Error("Parted channel but still getting messages", buf1)
-	}
-	if buf2.String() != buf2Expect.String() {
-		t.Error("Someone else parting somehow messed up sends", buf2)
-	}
+	c2.Expect(1, 22, 33, 44)
 }
diff --git a/cmd/vail/main.go b/cmd/vail/main.go
index 8e305ca..65194ee 100644
--- a/cmd/vail/main.go
+++ b/cmd/vail/main.go
@@ -12,6 +12,21 @@ import (
 
 var book Book
 
+// Clock defines an interface for getting the current time.
+//
+// We use this in testing to provide a fixed value for the current time, so we
+// can still compare clocks.
+type Clock interface {
+	Now() time.Time
+}
+
+// WallClock provides the actual time
+type WallClock struct{}
+
+func (WallClock) Now() time.Time {
+	return time.Now()
+}
+
 type Client struct {
 	repeaterName string
 }
@@ -22,9 +37,6 @@ func (c Client) Handle(ws *websocket.Conn) {
 	book.Join(c.repeaterName, ws)
 	defer book.Part(c.repeaterName, ws)
 
-	// Tell the client what time we think it is
-	fmt.Fprintf(ws, "[%d]", time.Now().UnixNano()/time.Millisecond.Nanoseconds())
-
 	for {
 		buf := make([]byte, ws.MaxPayloadBytes)
 
diff --git a/cmd/vail/message.go b/cmd/vail/message.go
index 5cadb7f..e35e7be 100644
--- a/cmd/vail/message.go
+++ b/cmd/vail/message.go
@@ -20,7 +20,7 @@ type Message struct {
 	Duration []uint8
 }
 
-func NewMessage(ts time.Time, durations []time.Duration) Message {
+func NewMessage(ts time.Time, durations ...time.Duration) Message {
 	msg := Message{
 		Timestamp: ts.UnixNano() / time.Millisecond.Nanoseconds(),
 		Duration:  make([]uint8, len(durations)),
diff --git a/cmd/vail/message_test.go b/cmd/vail/message_test.go
index 70ff0ec..dae2d44 100644
--- a/cmd/vail/message_test.go
+++ b/cmd/vail/message_test.go
@@ -40,11 +40,9 @@ func TestMessageStruct(t *testing.T) {
 			0,
 			m.Timestamp*time.Millisecond.Nanoseconds(),
 		),
-		[]time.Duration{
-			time.Duration(m.Duration[0]) * time.Millisecond,
-			time.Duration(m.Duration[1]) * time.Millisecond,
-			time.Duration(m.Duration[2]) * time.Millisecond,
-		},
+		time.Duration(m.Duration[0])*time.Millisecond,
+		time.Duration(m.Duration[1])*time.Millisecond,
+		time.Duration(m.Duration[2])*time.Millisecond,
 	)
 	if !m.Equal(m3) {
 		t.Error("NewMessage didn't work", m, m3)
diff --git a/cmd/vail/repeater.go b/cmd/vail/repeater.go
index 1fbd0b9..a2d00b8 100644
--- a/cmd/vail/repeater.go
+++ b/cmd/vail/repeater.go
@@ -3,23 +3,30 @@ package main
 import (
 	"io"
 	"log"
+	"time"
 )
 
 // A Repeater is just a list of Writers.
 type Repeater struct {
+	clock   Clock
 	writers []io.Writer
 }
 
+// NewRepeater returns a newly-created repeater
 func NewRepeater() *Repeater {
 	return &Repeater{
+		clock:   WallClock{},
 		writers: make([]io.Writer, 0, 20),
 	}
 }
 
+// Join joins a writer to this repeater
 func (r *Repeater) Join(w io.Writer) {
 	r.writers = append(r.writers, w)
+	r.SendMessage()
 }
 
+// Part removes a writer from this repeater
 func (r *Repeater) Part(w io.Writer) {
 	for i, s := range r.writers {
 		if s == w {
@@ -28,8 +35,10 @@ func (r *Repeater) Part(w io.Writer) {
 			r.writers = r.writers[:nsubs-1]
 		}
 	}
+	r.SendMessage()
 }
 
+// Send send a message to all connected clients
 func (r *Repeater) Send(m Message) {
 	m.Clients = uint16(r.Listeners())
 	buf, err := m.MarshalBinary()
@@ -41,6 +50,13 @@ func (r *Repeater) Send(m Message) {
 	}
 }
 
+// SendMessage constructs and sends a message
+func (r *Repeater) SendMessage(durations ...time.Duration) {
+	m := NewMessage(r.clock.Now(), durations...)
+	r.Send(m)
+}
+
+// Listeners returns the number of connected clients
 func (r *Repeater) Listeners() int {
 	return len(r.writers)
 }
diff --git a/cmd/vail/repeater_test.go b/cmd/vail/repeater_test.go
index 6d52b0c..66d2a32 100644
--- a/cmd/vail/repeater_test.go
+++ b/cmd/vail/repeater_test.go
@@ -2,57 +2,75 @@ package main
 
 import (
 	"bytes"
+	"io"
 	"testing"
+	"time"
 )
 
-type TestMessage struct {
-	Message
+type FakeClock struct{}
+
+func (f FakeClock) Now() time.Time {
+	return time.UnixMilli(0)
 }
 
-func (m TestMessage) bytes() []byte {
-	b, _ := m.MarshalBinary()
-	return b
+type TestingClient struct {
+	bytes.Buffer
+	expected bytes.Buffer
+	repeater *Repeater
+	t        *testing.T
+}
+
+func NewTestingClient(t *testing.T) *TestingClient {
+	return &TestingClient{
+		Buffer:   bytes.Buffer{},
+		expected: bytes.Buffer{},
+		t:        t,
+	}
+}
+
+func (tc *TestingClient) Expect(clients uint16, payload ...uint8) {
+	m := Message{0, clients, payload}
+	buf, _ := m.MarshalBinary()
+	tc.expected.Write(buf)
+	if tc.String() != tc.expected.String() {
+		tc.t.Errorf("Client buffer mismatch. Wanted %#v, got %#v", tc.expected.String(), tc.String())
+	}
+	tc.Reset()
+	tc.expected.Reset()
+}
+
+func NewTestingRepeater() *Repeater {
+	return &Repeater{
+		clock:   FakeClock{},
+		writers: make([]io.Writer, 0, 2),
+	}
 }
 
 func TestRepeater(t *testing.T) {
-	r := NewRepeater()
-	m := TestMessage{Message{1, 3, []uint8{3, 4}}}
+	r := NewTestingRepeater()
 
-	buf1 := bytes.NewBufferString("buf1")
-	buf1Expect := bytes.NewBufferString("buf1")
-	r.Join(buf1)
-	if r.Listeners() != 1 {
-		t.Error("Joining did nothing")
-	}
-	r.Send(m.Message)
-	m.Clients = 1
-	buf1Expect.Write(m.bytes())
-	if buf1.String() != buf1Expect.String() {
-		t.Error("Client 1 not repeating", buf1)
-	}
+	c1 := NewTestingClient(t)
+	r.Join(c1)
+	c1.Expect(1)
 
-	buf2 := bytes.NewBufferString("buf2")
-	buf2Expect := bytes.NewBufferString("buf2")
-	r.Join(buf2)
-	r.Send(m.Message)
-	m.Clients = 2
-	buf1Expect.Write(m.bytes())
-	buf2Expect.Write(m.bytes())
-	if buf1.String() != buf1Expect.String() {
-		t.Errorf("Client 1 not repeating %#v %#v", buf1, buf1Expect)
-	}
-	if buf2.String() != buf2Expect.String() {
-		t.Error("Client 2 not repeating", buf2)
-	}
+	r.SendMessage(15 * time.Millisecond)
+	c1.Expect(1, 15)
 
-	r.Part(buf1)
-	r.Send(m.Message)
-	m.Clients = 1
-	buf2Expect.Write(m.bytes())
-	if buf1.String() != buf1Expect.String() {
-		t.Error("Client 1 still getting data after part", buf1)
-	}
-	if buf2.String() != buf2Expect.String() {
-		t.Error("Client 2 not getting data after part", buf2)
+	c2 := NewTestingClient(t)
+	r.Join(c2)
+	c1.Expect(2)
+	c2.Expect(2)
+
+	r.SendMessage(58 * time.Millisecond)
+	c1.Expect(2, 58)
+	c2.Expect(2, 58)
+
+	r.Part(c1)
+	c2.Expect(1)
+
+	r.SendMessage(5 * time.Millisecond)
+	c2.Expect(1, 5)
+	if c1.Len() > 0 {
+		t.Error("Client 1 still getting data after part")
 	}
 }