diff --git a/tests/room_timestamp_to_event_test.go b/tests/room_timestamp_to_event_test.go
index 125f6606..fbc83e22 100644
--- a/tests/room_timestamp_to_event_test.go
+++ b/tests/room_timestamp_to_event_test.go
@@ -56,6 +56,9 @@ func TestJumpToDateEndpoint(t *testing.T) {
 		t.Run("should find nothing before the earliest timestamp", func(t *testing.T) {
 			t.Parallel()
 			timeBeforeRoomCreation := time.Now()
+			// Guard so createRoom cannot share this sample's millisecond; a
+			// backward search there would return m.room.create instead of nothing.
+			time.Sleep(tsBoundaryGuard)
 			roomID, _, _ := createTestRoom(t, alice)
 			mustCheckEventisReturnedForTime(t, alice, roomID, timeBeforeRoomCreation, "b", "")
 		})
@@ -76,6 +79,9 @@ func TestJumpToDateEndpoint(t *testing.T) {
 				deployment.GetFullyQualifiedHomeserverName(t, "hs1"),
 			})
 
+			// Guard so the join cannot share a millisecond with the messages below.
+			time.Sleep(tsBoundaryGuard)
+
 			// Send a couple messages with the same timestamp after the other test
 			// messages in the room.
 			timeBeforeMessageCreation := time.Now()
@@ -98,6 +104,9 @@ func TestJumpToDateEndpoint(t *testing.T) {
 				deployment.GetFullyQualifiedHomeserverName(t, "hs1"),
 			})
 
+			// Guard so the join cannot share a millisecond with the messages below.
+			time.Sleep(tsBoundaryGuard)
+
 			// Send a couple messages with the same timestamp after the other test
 			// messages in the room.
 			timeBeforeMessageCreation := time.Now()
@@ -113,6 +122,7 @@ func TestJumpToDateEndpoint(t *testing.T) {
 		t.Run("should not be able to query a private room you are not a member of", func(t *testing.T) {
 			t.Parallel()
 			timeBeforeRoomCreation := time.Now()
+			time.Sleep(tsBoundaryGuard)
 
 			// Alice will create the private room
 			roomID := alice.MustCreateRoom(t, map[string]interface{}{
@@ -141,6 +151,7 @@ func TestJumpToDateEndpoint(t *testing.T) {
 		t.Run("should not be able to query a public room you are not a member of", func(t *testing.T) {
 			t.Parallel()
 			timeBeforeRoomCreation := time.Now()
+			time.Sleep(tsBoundaryGuard)
 
 			// Alice will create the public room
 			roomID := alice.MustCreateRoom(t, map[string]interface{}{
@@ -187,6 +198,7 @@ func TestJumpToDateEndpoint(t *testing.T) {
 			t.Run("when looking backwards before the room was created, should be able to find event that was imported", func(t *testing.T) {
 				t.Parallel()
 				timeBeforeRoomCreation := time.Now()
+				time.Sleep(tsBoundaryGuard)
 				roomID, _, _ := createTestRoom(t, alice)
 
 				// Join from the application service bridge user so we can use it to send
@@ -331,6 +343,16 @@ type eventTime struct {
 	AfterTimestamp  time.Time
 }
 
+// tsBoundaryGuard is a pause inserted around (before and after) where we create events
+// so that `time.Now()` samples and subsequent event `origin_server_ts` don't collide at
+// the same millisecond granularity. /timestamp_to_event returns the boundary event
+// inclusively (forward picks the earliest event with ts >= query, backward picks the
+// latest with ts <= query), so a shared millisecond between events means the wrong
+// event can be picked. Adding one whole millisecond to a timestamp always carries it
+// into the next millisecond bucket, so 1ms is enough to separate the sample from every
+// event stamped after the pause.
+const tsBoundaryGuard = 1 * time.Millisecond
+
 func createTestRoom(t *testing.T, c *client.CSAPI) (roomID string, eventA, eventB *eventTime) {
 	t.Helper()
 
@@ -339,6 +361,7 @@ func createTestRoom(t *testing.T, c *client.CSAPI) (roomID string, eventA, event
 	})
 
 	timeBeforeEventA := time.Now()
+	time.Sleep(tsBoundaryGuard)
 	eventAID := c.SendEventSynced(t, roomID, b.Event{
 		Type: "m.room.message",
 		Content: map[string]interface{}{
@@ -346,8 +369,12 @@ func createTestRoom(t *testing.T, c *client.CSAPI) (roomID string, eventA, event
 			"body":    "Message A",
 		},
 	})
-	timeAfterEventA := time.Now()
 
+	// timeBeforeEventB doubles as eventA's after-timestamp, so guard it on
+	// both sides to keep it between the two events.
+	time.Sleep(tsBoundaryGuard)
+	timeBeforeEventB := time.Now()
+	time.Sleep(tsBoundaryGuard)
 	eventBID := c.SendEventSynced(t, roomID, b.Event{
 		Type: "m.room.message",
 		Content: map[string]interface{}{
@@ -355,10 +382,12 @@ func createTestRoom(t *testing.T, c *client.CSAPI) (roomID string, eventA, event
 			"body":    "Message B",
 		},
 	})
+
+	time.Sleep(tsBoundaryGuard)
 	timeAfterEventB := time.Now()
 
-	eventA = &eventTime{EventID: eventAID, BeforeTimestamp: timeBeforeEventA, AfterTimestamp: timeAfterEventA}
-	eventB = &eventTime{EventID: eventBID, BeforeTimestamp: timeAfterEventA, AfterTimestamp: timeAfterEventB}
+	eventA = &eventTime{EventID: eventAID, BeforeTimestamp: timeBeforeEventA, AfterTimestamp: timeBeforeEventB}
+	eventB = &eventTime{EventID: eventBID, BeforeTimestamp: timeBeforeEventB, AfterTimestamp: timeAfterEventB}
 
 	return roomID, eventA, eventB
 }