diff --git a/app.py b/app.py
index aa61d15..cdfa4ef 100644
--- a/app.py
+++ b/app.py
@@ -85,7 +85,7 @@ def results(slug, travel_date):
 
     cached_rtt = get_cached(rtt_cache_key)
     cached_es = get_cached(es_cache_key)
-    cached_prices = get_cached(prices_cache_key)
+    cached_prices = get_cached(prices_cache_key, ttl=24 * 3600)
     from_cache = bool(cached_rtt and cached_es and cached_prices)
 
     error = None
diff --git a/cache.py b/cache.py
index 668103f..2e82500 100644
--- a/cache.py
+++ b/cache.py
@@ -1,5 +1,6 @@
 import json
 import os
+import time
 
 CACHE_DIR = os.path.join(os.path.dirname(__file__), 'cache')
 
@@ -9,10 +10,13 @@ def _cache_path(key: str) -> str:
     return os.path.join(CACHE_DIR, f"{safe_key}.json")
 
 
-def get_cached(key: str):
+def get_cached(key: str, ttl: int | None = None):
+    """Return cached data, or None if missing or older than ttl seconds."""
     path = _cache_path(key)
     if not os.path.exists(path):
         return None
+    if ttl is not None and time.time() - os.path.getmtime(path) > ttl:
+        return None
     with open(path) as f:
         return json.load(f)
 
diff --git a/tests/test_app.py b/tests/test_app.py
index 4b8261e..cd21232 100644
--- a/tests/test_app.py
+++ b/tests/test_app.py
@@ -7,7 +7,7 @@ def _client():
 
 
 def _stub_data(monkeypatch, prices=None):
-    monkeypatch.setattr(app_module, 'get_cached', lambda key: None)
+    monkeypatch.setattr(app_module, 'get_cached', lambda key, ttl=None: None)
     monkeypatch.setattr(app_module, 'set_cached', lambda key, data: None)
     monkeypatch.setattr(
         app_module.rtt_scraper,
@@ -94,7 +94,7 @@ def test_results_title_and_social_meta_include_destination(monkeypatch):
 
 
 def test_results_marks_trips_within_five_minutes_of_fastest_and_slowest(monkeypatch):
-    monkeypatch.setattr(app_module, 'get_cached', lambda key: None)
+    monkeypatch.setattr(app_module, 'get_cached', lambda key, ttl=None: None)
     monkeypatch.setattr(app_module, 'set_cached', lambda key, data: None)
     monkeypatch.setattr(app_module, 'fetch_eurostar_prices', lambda dest, date: {})
     monkeypatch.setattr(
@@ -166,7 +166,7 @@ def test_results_marks_trips_within_five_minutes_of_fastest_and_slowest(monkeypa
 
 
 def test_results_shows_unreachable_morning_eurostar_services(monkeypatch):
-    monkeypatch.setattr(app_module, 'get_cached', lambda key: None)
+    monkeypatch.setattr(app_module, 'get_cached', lambda key, ttl=None: None)
     monkeypatch.setattr(app_module, 'set_cached', lambda key, data: None)
     monkeypatch.setattr(app_module, 'fetch_eurostar_prices', lambda dest, date: {})
     monkeypatch.setattr(
@@ -233,7 +233,7 @@ def test_results_shows_eurostar_price_and_total(monkeypatch):
 
 
 def test_results_can_show_only_unreachable_morning_services(monkeypatch):
-    monkeypatch.setattr(app_module, 'get_cached', lambda key: None)
+    monkeypatch.setattr(app_module, 'get_cached', lambda key, ttl=None: None)
     monkeypatch.setattr(app_module, 'set_cached', lambda key, data: None)
     monkeypatch.setattr(app_module, 'fetch_eurostar_prices', lambda dest, date: {})
     monkeypatch.setattr(
diff --git a/tests/test_cache.py b/tests/test_cache.py
new file mode 100644
index 0000000..006af57
--- /dev/null
+++ b/tests/test_cache.py
@@ -0,0 +1,42 @@
+import os
+import time
+import pytest
+from cache import get_cached, set_cached
+
+
+@pytest.fixture
+def tmp_cache(tmp_path, monkeypatch):
+    import cache as cache_module
+    monkeypatch.setattr(cache_module, 'CACHE_DIR', str(tmp_path))
+    return tmp_path
+
+
+def test_get_cached_returns_none_for_missing_key(tmp_cache):
+    assert get_cached('no_such_key') is None
+
+
+def test_set_and_get_cached_roundtrip(tmp_cache):
+    set_cached('my_key', {'a': 1})
+    assert get_cached('my_key') == {'a': 1}
+
+
+def test_get_cached_no_ttl_never_expires(tmp_cache):
+    set_cached('k', [1, 2, 3])
+    # Backdate the file by 2 days
+    path = tmp_cache / 'k.json'
+    old = time.time() - 2 * 86400
+    os.utime(path, (old, old))
+    assert get_cached('k') == [1, 2, 3]
+
+
+def test_get_cached_within_ttl(tmp_cache):
+    set_cached('k', 'fresh')
+    assert get_cached('k', ttl=3600) == 'fresh'
+
+
+def test_get_cached_expired_returns_none(tmp_cache):
+    set_cached('k', 'stale')
+    path = tmp_cache / 'k.json'
+    old = time.time() - 25 * 3600  # 25 hours ago
+    os.utime(path, (old, old))
+    assert get_cached('k', ttl=24 * 3600) is None