diff --git a/src/slack_annotations/core.py b/src/slack_annotations/core.py
index 7799182..9176688 100644
--- a/src/slack_annotations/core.py
+++ b/src/slack_annotations/core.py
@@ -14,10 +14,7 @@ def notify(
     # values are needed for the algorithm below to work.
     search_params["sort"] = "created"
     search_params["order"] = "asc"
-    if not search_params.get("search_after"):
-        search_params["search_after"] = (
-            datetime.now(UTC) - timedelta(hours=1)
-        ).isoformat()
+    search_params["search_after"] = (datetime.now(UTC) - timedelta(hours=1)).isoformat()
 
     if cache_path:
         try:
diff --git a/tests/functional/core_test.py b/tests/functional/core_test.py
index ba1ce45..4e5a07e 100644
--- a/tests/functional/core_test.py
+++ b/tests/functional/core_test.py
@@ -1,38 +1,42 @@
 import json
+from urllib.parse import quote
+
 import pytest
+from freezegun import freeze_time
 
 from slack_annotations.core import notify
 
 
 @pytest.fixture
 def slack_annotations():
-    with open("tests/data/slack_annotations.json") as f:
+    with open("tests/data/slack_annotations.json", encoding="utf-8") as f:
         return json.load(f)
 
 
 @pytest.fixture
 def search_annotations():
-    with open("tests/data/search_annotations.json") as f:
+    with open("tests/data/search_annotations.json", encoding="utf-8") as f:
         return json.load(f)
 
 
+@freeze_time("2024-12-01T01:00:00+00:00")
 def test_notify_search_after(httpx_mock, search_annotations, slack_annotations):
-    url = "https://hypothes.is/api/search?search_after=1733007600000.0&sort=created&order=asc"
+    search_after = quote("2024-12-01T00:00:00+00:00")
+    url = f"https://hypothes.is/api/search?sort=created&order=asc&search_after={search_after}"
     httpx_mock.add_response(url=url, content=json.dumps(search_annotations))
 
-    search_params = {"search_after": "1733007600000.0"}
-    assert notify(search_params=search_params) == json.dumps(slack_annotations)
+    assert notify() == json.dumps(slack_annotations)
 
 
+@freeze_time("2024-11-01T01:00:00+00:00")
 def test_notify_cache_file(httpx_mock, tmp_path, search_annotations, slack_annotations):
-    url = "https://hypothes.is/api/search?search_after=1733007600000.0&sort=created&order=asc"
+    search_after = quote("2024-12-01T00:00:00+00:00")
+    url = f"https://hypothes.is/api/search?sort=created&order=asc&search_after={search_after}"
     httpx_mock.add_response(url=url, content=json.dumps(search_annotations))
 
-    search_params = {"search_after": "1733007600000.0"}
     cache_path = tmp_path / "cache.json"
-    assert notify(search_params=search_params, cache_path=cache_path) == json.dumps(
-        slack_annotations
-    )
+    cache_path.write_text(json.dumps({"search_after": "2024-12-01T00:00:00+00:00"}))
+    assert notify(cache_path=cache_path) == json.dumps(slack_annotations)
     assert json.loads(cache_path.read_text()) == {
         "search_after": "2024-11-22T17:03:54.248643+00:00"
     }
diff --git a/tox.ini b/tox.ini
index 435b78c..706e73d 100644
--- a/tox.ini
+++ b/tox.ini
@@ -38,6 +38,7 @@ deps =
     lint,tests,functests: pytest
     lint,tests,functests: h-testkit
     lint,functests: pytest-httpx
+    lint,functests: freezegun
     tests: pytest-cov
     coverage: coverage[toml]
     lint,tests,functests: factory-boy