diff --git a/circuitbreaker/breaker.go b/circuitbreaker/breaker.go
index b4f2629..7a26fab 100644
--- a/circuitbreaker/breaker.go
+++ b/circuitbreaker/breaker.go
@@ -72,7 +72,7 @@ func (b *Breaker) State() State {
 // stateLocked returns the effective state, promoting Open → HalfOpen when the
 // open duration has elapsed. Caller must hold b.mu.
 func (b *Breaker) stateLocked() State {
-	if b.state == StateOpen && time.Since(b.openedAt) >= b.opts.openDuration {
+	if b.state == StateOpen && b.opts.clk.Since(b.openedAt) >= b.opts.openDuration {
 		b.state = StateHalfOpen
 		b.halfOpenCur = 0
 	}
@@ -142,7 +142,7 @@ func (b *Breaker) record(success bool) {
 // tripLocked transitions to the Open state and records the timestamp.
 func (b *Breaker) tripLocked() {
 	b.state = StateOpen
-	b.openedAt = time.Now()
+	b.openedAt = b.opts.clk.Now()
 	b.halfOpenCur = 0
 }
 
diff --git a/circuitbreaker/breaker_test.go b/circuitbreaker/breaker_test.go
index 0388a37..7781100 100644
--- a/circuitbreaker/breaker_test.go
+++ b/circuitbreaker/breaker_test.go
@@ -8,6 +8,7 @@ import (
 	"testing"
 	"time"
 
+	"git.codelab.vc/pkg/httpx/internal/clock"
 	"git.codelab.vc/pkg/httpx/middleware"
 )
 
@@ -80,9 +81,11 @@ func TestBreaker_OpenRejectsRequests(t *testing.T) {
 
 func TestBreaker_TransitionsToHalfOpenAfterDuration(t *testing.T) {
 	const openDuration = 50 * time.Millisecond
+	clk := clock.Mock(time.Now())
 	b := NewBreaker(
 		WithFailureThreshold(1),
 		WithOpenDuration(openDuration),
+		withClock(clk),
 	)
 
 	// Trip the breaker.
@@ -96,8 +99,8 @@ func TestBreaker_TransitionsToHalfOpenAfterDuration(t *testing.T) {
 		t.Fatalf("state = %v, want %v", s, StateOpen)
 	}
 
-	// Wait for the open duration to elapse.
-	time.Sleep(openDuration + 10*time.Millisecond)
+	// Advance past the open duration.
+	clk.Advance(openDuration + time.Millisecond)
 
 	if s := b.State(); s != StateHalfOpen {
 		t.Fatalf("state = %v, want %v", s, StateHalfOpen)
@@ -106,9 +109,11 @@ func TestBreaker_TransitionsToHalfOpenAfterDuration(t *testing.T) {
 
 func TestBreaker_HalfOpenToClosedOnSuccess(t *testing.T) {
 	const openDuration = 50 * time.Millisecond
+	clk := clock.Mock(time.Now())
 	b := NewBreaker(
 		WithFailureThreshold(1),
 		WithOpenDuration(openDuration),
+		withClock(clk),
 	)
 
 	// Trip the breaker.
@@ -118,8 +123,8 @@ func TestBreaker_HalfOpenToClosedOnSuccess(t *testing.T) {
 	}
 	done(false)
 
-	// Wait for half-open.
-	time.Sleep(openDuration + 10*time.Millisecond)
+	// Advance into half-open.
+	clk.Advance(openDuration + time.Millisecond)
 
 	// A successful request in half-open should close the breaker.
 	done, err = b.Allow()
@@ -135,9 +140,11 @@ func TestBreaker_HalfOpenToClosedOnSuccess(t *testing.T) {
 
 func TestBreaker_HalfOpenToOpenOnFailure(t *testing.T) {
 	const openDuration = 50 * time.Millisecond
+	clk := clock.Mock(time.Now())
 	b := NewBreaker(
 		WithFailureThreshold(1),
 		WithOpenDuration(openDuration),
+		withClock(clk),
 	)
 
 	// Trip the breaker.
@@ -147,8 +154,8 @@ func TestBreaker_HalfOpenToOpenOnFailure(t *testing.T) {
 	}
 	done(false)
 
-	// Wait for half-open.
-	time.Sleep(openDuration + 10*time.Millisecond)
+	// Advance into half-open.
+	clk.Advance(openDuration + time.Millisecond)
 
 	// A failed request in half-open should re-open the breaker.
 	done, err = b.Allow()
diff --git a/circuitbreaker/options.go b/circuitbreaker/options.go
index 57bd770..bcfdfc0 100644
--- a/circuitbreaker/options.go
+++ b/circuitbreaker/options.go
@@ -1,11 +1,16 @@
 package circuitbreaker
 
-import "time"
+import (
+	"time"
+
+	"git.codelab.vc/pkg/httpx/internal/clock"
+)
 
 type options struct {
 	failureThreshold int           // consecutive failures to trip
 	openDuration     time.Duration // how long to stay open before half-open
 	halfOpenMax      int           // max concurrent requests in half-open
+	clk              clock.Clock   // time source (real by default)
 }
 
 func defaults() options {
@@ -13,12 +18,23 @@ func defaults() options {
 		failureThreshold: 5,
 		openDuration:     30 * time.Second,
 		halfOpenMax:      1,
+		clk:              clock.System(),
 	}
 }
 
 // Option configures a Breaker.
 type Option func(*options)
 
+// withClock sets the clock used for state-transition timing. Unexported; for
+// deterministic tests.
+func withClock(c clock.Clock) Option {
+	return func(o *options) {
+		if c != nil {
+			o.clk = c
+		}
+	}
+}
+
 // WithFailureThreshold sets the number of consecutive failures required to
 // trip the breaker from Closed to Open. Default is 5.
 func WithFailureThreshold(n int) Option {