diff --git a/src/isobmff/isobmff-demuxer.ts b/src/isobmff/isobmff-demuxer.ts
index 6f5fd59..5e4fbf5 100644
--- a/src/isobmff/isobmff-demuxer.ts
+++ b/src/isobmff/isobmff-demuxer.ts
@@ -2196,6 +2196,7 @@ abstract class IsobmffTrackBacking implements InputTrackBacking {
 
 	private findSampleInFragmentsForTimestamp(timestampInTimescale: number) {
 		const fragmentIndex = binarySearchLessOrEqual(
+			// This array is technically not sorted by start timestamp, but for any reasonable file, it basically is.
 			this.internalTrack.fragments,
 			timestampInTimescale,
 			x => x.trackData.get(this.internalTrack.id)!.startTimestamp,
@@ -2223,6 +2224,7 @@ abstract class IsobmffTrackBacking implements InputTrackBacking {
 
 	private findKeySampleInFragmentsForTimestamp(timestampInTimescale: number) {
 		const fragmentIndex = binarySearchLessOrEqual(
+			// This array is technically not sorted by start timestamp, but for any reasonable file, it basically is.
 			this.internalTrack.fragments,
 			timestampInTimescale,
 			x => x.trackData.get(this.internalTrack.id)!.startTimestamp,
diff --git a/src/matroska/matroska-demuxer.ts b/src/matroska/matroska-demuxer.ts
index 64d9d9d..074dbaa 100644
--- a/src/matroska/matroska-demuxer.ts
+++ b/src/matroska/matroska-demuxer.ts
@@ -507,16 +507,16 @@ export class MatroskaDemuxer extends Demuxer {
 			if (track) {
 				const insertionIndex = binarySearchLessOrEqual(
 					track.clusters,
-					cluster.timestamp,
-					x => x.timestamp,
+					cluster.elementStartPos,
+					x => x.elementStartPos,
 				);
 				track.clusters.splice(insertionIndex + 1, 0, cluster);
 
 				if (hasKeyFrame) {
 					const insertionIndex = binarySearchLessOrEqual(
 						track.clustersWithKeyFrame,
-						cluster.timestamp,
-						x => x.timestamp,
+						cluster.elementStartPos,
+						x => x.elementStartPos,
 					);
 					track.clustersWithKeyFrame.splice(insertionIndex + 1, 0, cluster);
 				}
@@ -1268,6 +1268,7 @@ abstract class MatroskaTrackBacking implements InputTrackBacking {
 
 	private findBlockInClustersForTimestamp(timestampInTimescale: number) {
 		const clusterIndex = binarySearchLessOrEqual(
+			// This array is technically not sorted by start timestamp, but for any reasonable file, it basically is.
 			this.internalTrack.clusters,
 			timestampInTimescale,
 			x => x.trackData.get(this.internalTrack.id)!.startTimestamp,
@@ -1295,6 +1296,7 @@ abstract class MatroskaTrackBacking implements InputTrackBacking {
 
 	private findKeyBlockInClustersForTimestamp(timestampInTimescale: number) {
 		const indexInKeyFrameClusters = binarySearchLessOrEqual(
+			// This array is technically not sorted by start timestamp, but for any reasonable file, it basically is.
 			this.internalTrack.clustersWithKeyFrame,
 			timestampInTimescale,
 			x => x.trackData.get(this.internalTrack.id)!.firstKeyFrameTimestamp!,
diff --git a/src/misc.ts b/src/misc.ts
index 854f4f2..9b07f4b 100644
--- a/src/misc.ts
+++ b/src/misc.ts
@@ -233,17 +233,18 @@ export const reverseBitsU32 = (x: number): number => {
 	return x >>> 0; // Ensure it's treated as an unsigned 32-bit integer
 };
 
+/** Returns the smallest index i such that val[i] === key, or -1 if no such index exists. */
 export const binarySearchExact = <T>(arr: T[], key: number, valueGetter: (x: T) => number): number => {
 	let low = 0;
 	let high = arr.length - 1;
-	let res = -1;
+	let ans = -1;
 
 	while (low <= high) {
 		const mid = (low + high) >> 1;
 		const midVal = valueGetter(arr[mid]!);
 
 		if (midVal === key) {
-			res = mid;
+			ans = mid;
 			high = mid - 1; // Continue searching left to find the lowest index
 		} else if (midVal < key) {
 			low = mid + 1;
@@ -252,13 +253,14 @@ export const binarySearchExact = <T>(arr: T[], key: number, valueGetter: (x: T)
 		}
 	}
 
-	return res;
+	return ans;
 };
 
+/** Returns the largest index i such that val[i] <= key, or -1 if no such index exists. */
 export const binarySearchLessOrEqual = <T>(arr: T[], key: number, valueGetter: (x: T) => number) => {
-	let ans = -1;
 	let low = 0;
 	let high = arr.length - 1;
+	let ans = -1;
 
 	while (low <= high) {
 		const mid = (low + (high - low + 1) / 2) | 0;