diff --git a/WebHostLib/tracker.py b/WebHostLib/tracker.py
index 75b5fb02..5450ef51 100644
--- a/WebHostLib/tracker.py
+++ b/WebHostLib/tracker.py
@@ -5,7 +5,7 @@ from typing import Any, Callable, Dict, List, Optional, Set, Tuple, NamedTuple,
 from uuid import UUID
 from email.utils import parsedate_to_datetime
 
-from flask import render_template, make_response, Response, request
+from flask import make_response, render_template, request, Request, Response
 from werkzeug.exceptions import abort
 
 from MultiServer import Context, get_saving_second
@@ -298,17 +298,25 @@ class TrackerData:
         return self._multidata.get("spheres", [])
 
 
-def _process_if_request_valid(incoming_request, room: Optional[Room]) -> Optional[Response]:
+def _process_if_request_valid(incoming_request: Request, room: Optional[Room]) -> Optional[Response]:
     if not room:
         abort(404)
 
-    if_modified = incoming_request.headers.get("If-Modified-Since", None)
-    if if_modified:
-        if_modified = parsedate_to_datetime(if_modified)
+    if_modified_str: Optional[str] = incoming_request.headers.get("If-Modified-Since", None)
+    if if_modified_str:
+        if_modified = parsedate_to_datetime(if_modified_str)
+        if if_modified.tzinfo is None:
+            abort(400)  # standard requires "GMT" timezone
+        # database may use datetime.utcnow(), which is timezone-naive. convert to timezone-aware.
+        last_activity = room.last_activity
+        if last_activity.tzinfo is None:
+            last_activity = room.last_activity.replace(tzinfo=datetime.timezone.utc)
         # if_modified has less precision than last_activity, so we bring them to same precision
-        if if_modified >= room.last_activity.replace(microsecond=0):
+        if if_modified >= last_activity.replace(microsecond=0):
             return make_response("",  304)
 
+    return None
+
 
 @app.route("/tracker/<suuid:tracker>/<int:tracked_team>/<int:tracked_player>")
 def get_player_tracker(tracker: UUID, tracked_team: int, tracked_player: int, generic: bool = False) -> Response:
diff --git a/test/webhost/data/One_Archipelago.archipelago b/test/webhost/data/One_Archipelago.archipelago
new file mode 100644
index 00000000..8b7a8ce0
Binary files /dev/null and b/test/webhost/data/One_Archipelago.archipelago differ
diff --git a/test/webhost/test_tracker.py b/test/webhost/test_tracker.py
new file mode 100644
index 00000000..58145d77
--- /dev/null
+++ b/test/webhost/test_tracker.py
@@ -0,0 +1,95 @@
+import os
+import pickle
+from pathlib import Path
+from typing import ClassVar
+from uuid import UUID, uuid4
+
+from flask import url_for
+
+from . import TestBase
+
+
+class TestTracker(TestBase):
+    room_id: UUID
+    tracker_uuid: UUID
+    log_filename: str
+    data: ClassVar[bytes]
+
+    @classmethod
+    def setUpClass(cls) -> None:
+        super().setUpClass()
+        with (Path(__file__).parent / "data" / "One_Archipelago.archipelago").open("rb") as f:
+            cls.data = f.read()
+
+    def setUp(self) -> None:
+        from pony.orm import db_session
+        from MultiServer import Context as MultiServerContext
+        from Utils import user_path
+        from WebHostLib.models import GameDataPackage, Room, Seed
+
+        super().setUp()
+
+        multidata = MultiServerContext.decompress(self.data)
+
+        with self.client.session_transaction() as session:
+            session["_id"] = uuid4()
+            self.tracker_uuid = uuid4()
+            with db_session:
+                # store game datapackage(s)
+                for game, game_data in multidata["datapackage"].items():
+                    if not GameDataPackage.get(checksum=game_data["checksum"]):
+                        GameDataPackage(checksum=game_data["checksum"],
+                                        data=pickle.dumps(game_data))
+                # create an empty seed and a room from it
+                seed = Seed(multidata=self.data, owner=session["_id"])
+                room = Room(seed=seed, owner=session["_id"], tracker=self.tracker_uuid)
+                self.room_id = room.id
+                self.log_filename = user_path("logs", f"{self.room_id}.txt")
+
+    def tearDown(self) -> None:
+        from pony.orm import db_session, select
+        from WebHostLib.models import Command, Room
+
+        with db_session:
+            for command in select(command for command in Command if command.room.id == self.room_id):  # type: ignore
+                command.delete()
+            room: Room = Room.get(id=self.room_id)
+            room.seed.delete()
+            room.delete()
+
+        try:
+            os.unlink(self.log_filename)
+        except FileNotFoundError:
+            pass
+
+    def test_valid_if_modified_since(self) -> None:
+        """
+        Verify that we get a 200 response for valid If-Modified-Since
+        """
+        with self.app.app_context(), self.app.test_request_context():
+            response = self.client.get(
+                url_for(
+                    "get_player_tracker",
+                    tracker=self.tracker_uuid,
+                    tracked_team=0,
+                    tracked_player=1,
+                ),
+                headers={"If-Modified-Since": "Wed, 21 Oct 2015 07:28:00 GMT"},
+            )
+            self.assertEqual(response.status_code, 200)
+
+    def test_invalid_if_modified_since(self) -> None:
+        """
+        Verify that we get a 400 response for invalid If-Modified-Since
+        """
+        with self.app.app_context(), self.app.test_request_context():
+            response = self.client.get(
+                url_for(
+                    "get_player_tracker",
+                    tracker=self.tracker_uuid,
+                    tracked_team=1,
+                    tracked_player=0,
+                ),
+                headers={"If-Modified-Since": "Wed, 21 Oct 2015 07:28:00"},  # missing timezone
+            )
+            self.assertEqual(response.status_code, 400)