diff --git a/frontend/src/lib/document/timeline.ts b/frontend/src/lib/document/timeline.ts
index 81d5c90e..0653ea99 100644
--- a/frontend/src/lib/document/timeline.ts
+++ b/frontend/src/lib/document/timeline.ts
@@ -61,6 +61,9 @@ export function fillDensityGaps(
  * `[fromInclusive, toInclusive]` ISO date range. When either bound is null the
  * input array is returned unchanged. Used by the timeline's zoom-in tool to
  * narrow the visible bars without refetching data.
+ *
+ * @internal Sole call site is `TimelineDensityFilter.svelte`. Exported so the
+ * unit suite (`timeline.spec.ts`) can pin the boundary semantics directly.
  */
 export function clipBucketsToRange(
 	buckets: MonthBucket[],