diff --git a/web/src/utils/timelineUtil.tsx b/web/src/utils/timelineUtil.tsx
index 502121471..466dc7dd9 100644
--- a/web/src/utils/timelineUtil.tsx
+++ b/web/src/utils/timelineUtil.tsx
@@ -38,6 +38,22 @@ export function getChunkedTimeDay(timeRange: TimeRange): TimeRange[] {
   return data;
 }
 
+/**
+ * Find the chunk index that contains the given timestamp.
+ * Uses half-open intervals [after, before) for all chunks except the last,
+ * which uses a closed interval [after, before] so the terminal boundary
+ * is always reachable.
+ */
+export function findChunkIndex(chunks: TimeRange[], timestamp: number): number {
+  return chunks.findIndex((chunk, i) => {
+    const isLast = i === chunks.length - 1;
+    return (
+      chunk.after <= timestamp &&
+      (isLast ? chunk.before >= timestamp : chunk.before > timestamp)
+    );
+  });
+}
+
 export function getChunkedTimeRange(
   startTimestamp: number,
   endTimestamp: number,
diff --git a/web/src/views/recording/RecordingView.tsx b/web/src/views/recording/RecordingView.tsx
index 0445a477b..7f382dee1 100644
--- a/web/src/views/recording/RecordingView.tsx
+++ b/web/src/views/recording/RecordingView.tsx
@@ -26,7 +26,7 @@ import {
   ReviewSummary,
   ZoomLevel,
 } from "@/types/review";
-import { getChunkedTimeDay } from "@/utils/timelineUtil";
+import { findChunkIndex, getChunkedTimeDay } from "@/utils/timelineUtil";
 import {
   MutableRefObject,
   useCallback,
@@ -169,9 +169,7 @@ export function RecordingView({
     [timeRange],
   );
   const [selectedRangeIdx, setSelectedRangeIdx] = useState(
-    chunkedTimeRange.findIndex((chunk) => {
-      return chunk.after <= startTime && chunk.before > startTime;
-    }),
+    findChunkIndex(chunkedTimeRange, startTime),
   );
   const currentTimeRange = useMemo<TimeRange>(
     () =>
@@ -274,9 +272,7 @@ export function RecordingView({
 
   const updateSelectedSegment = useCallback(
     (currentTime: number, updateStartTime: boolean) => {
-      const index = chunkedTimeRange.findIndex(
-        (seg) => seg.after <= currentTime && seg.before > currentTime,
-      );
+      const index = findChunkIndex(chunkedTimeRange, currentTime);
 
       if (index != -1) {
         if (updateStartTime) {