diff --git a/README.md b/README.md
index 0ebad92..d23de3f 100644
--- a/README.md
+++ b/README.md
@@ -105,6 +105,7 @@ type Result[V any] struct {
 | `NegativeTTL` | How long an authoritative miss is served from cache before re-fetching. `0` disables negative caching (misses still evict). |
 | `OnError` | Optional `func(key any, err error)` called when a background refresh fails. |
 | `Context` | Base context for background refreshes (they outlive the request). Defaults to `context.Background()`. |
+| `Clock` | Optional `func() time.Time` used for all expiry checks. Defaults to `time.Now`. Override in tests to control TTLs deterministically; must be safe for concurrent use. |
 
 ## Migrating from v1
 
diff --git a/lastcache.go b/lastcache.go
index b3516d5..a13e4df 100644
--- a/lastcache.go
+++ b/lastcache.go
@@ -80,6 +80,12 @@ type Config struct {
 	// Context is the base context used for background refreshes, which outlive
 	// the request that triggered them. Defaults to context.Background().
 	Context context.Context
+
+	// Clock returns the current time and defaults to time.Now. Override it in
+	// tests to control TTL expiry deterministically instead of sleeping. The
+	// cache reads it from background goroutines too, so a test clock must be
+	// safe for concurrent use.
+	Clock func() time.Time
 }
 
 // Result carries a value plus whether it was served stale.
@@ -140,9 +146,14 @@ func New[K comparable, V any](config Config) *Cache[K, V] {
 		base = context.Background()
 	}
 
+	clock := config.Clock
+	if clock == nil {
+		clock = time.Now
+	}
+
 	return &Cache[K, V]{
 		config:    config,
-		clock:     time.Now,
+		clock:     clock,
 		baseCtx:   base,
 		entries:   make(map[K]item[V]),
 		semaphore: make(chan struct{}, sem),
diff --git a/lastcache_test.go b/lastcache_test.go
index 93c25a3..4591633 100644
--- a/lastcache_test.go
+++ b/lastcache_test.go
@@ -34,6 +34,40 @@ func TestNew_Defaults(t *testing.T) {
 	}
 }
 
+// Config.Clock lets tests control expiry without sleeping or touching
+// internals.
+func TestConfig_Clock(t *testing.T) {
+	var mu sync.Mutex
+	now := fixedTime
+	c := New[string, string](Config{
+		TTL: time.Minute,
+		Clock: func() time.Time {
+			mu.Lock()
+			defer mu.Unlock()
+			return now
+		},
+	})
+	c.Set("k", "v")
+
+	var calls int32
+	fetch := func(context.Context, string) (string, error) {
+		atomic.AddInt32(&calls, 1)
+		return "v2", nil
+	}
+
+	if v, _ := c.Get(context.Background(), "k", fetch); v != "v" || atomic.LoadInt32(&calls) != 0 {
+		t.Fatalf("fresh hit: got %q with %d fetches, want \"v\" with 0", v, atomic.LoadInt32(&calls))
+	}
+
+	mu.Lock()
+	now = now.Add(2 * time.Minute) // expire
+	mu.Unlock()
+
+	if v, _ := c.Get(context.Background(), "k", fetch); v != "v2" || atomic.LoadInt32(&calls) != 1 {
+		t.Fatalf("after expiry: got %q with %d fetches, want \"v2\" with 1", v, atomic.LoadInt32(&calls))
+	}
+}
+
 func TestGet_FreshHit_NoFetch(t *testing.T) {
 	now := fixedTime
 	c := New[string, int](Config{TTL: time.Minute})