diff --git a/src/conversion.ts b/src/conversion.ts
index 2c010cd..f3ad24c 100644
--- a/src/conversion.ts
+++ b/src/conversion.ts
@@ -1781,8 +1781,16 @@ export class Conversion {
 						break;
 					}
 
-					const adjustedSampleTimestamp = Math.max(0, sample.timestamp + this._timestampOffset);
-					sample.setTimestamp(adjustedSampleTimestamp);
+					const clampedStartTimestamp = Math.max(this._startTimestamp, sample.timestamp);
+					const clampedEndTimestamp = Math.min(this._endTimestamp, sample.timestamp + sample.duration);
+
+					if (clampedStartTimestamp >= clampedEndTimestamp) {
+						// Wholly out of the trim region
+						continue;
+					}
+
+					sample.setTimestamp(clampedStartTimestamp + this._timestampOffset);
+					sample.setDuration(clampedEndTimestamp - clampedStartTimestamp);
 
 					this._reportProgress(outputTrackId, sample.timestamp + sample.duration);
 					await source.add(sample);
@@ -1874,8 +1882,17 @@ export class Conversion {
 
 			if (
 				startPacket
-				&& startPacket.timestamp < this._startTimestamp
-				&& this._copyBoundaryPolicy === 'shrink'
+				&& (
+					(
+						this._copyBoundaryPolicy === 'shrink'
+						&& startPacket.timestamp < this._startTimestamp
+					)
+					|| (
+						this._copyBoundaryPolicy === 'expand'
+						// Check if packet is wholly before the start
+						&& startPacket.timestamp + startPacket.duration <= this._startTimestamp
+					)
+				)
 			) {
 				startPacket = await sink.getNextKeyPacket(startPacket);
 			}
diff --git a/test/browser/conversion.test.ts b/test/browser/conversion.test.ts
index 05e98b9..9a24e85 100644
--- a/test/browser/conversion.test.ts
+++ b/test/browser/conversion.test.ts
@@ -939,8 +939,8 @@ test('Packet copy, delta frame trim, video transcoded, Matroska', async () => {
 				shiftTolerance: Infinity,
 			},
 		},
-		expectedTimeOffset: 1.484,
-		videoStartTimestamp: 0.008,
+		expectedTimeOffset: 1.472,
+		videoStartTimestamp: 0.028,
 		videoEndTimestamp: 1.008,
 		audioStartTimestamp: 0,
 		audioEndTimestamp: 1.024,
@@ -1053,3 +1053,130 @@ const testCopy = async (options: {
 		}
 	}
 };
+
+test('Trim wholly before media data, transcode', async () => {
+	using input = new Input({
+		source: new UrlSource('/demo.mp4'),
+		formats: ALL_FORMATS,
+	});
+	const output = new Output({
+		format: new Mp4OutputFormat(),
+		target: new BufferTarget(),
+	});
+
+	const conversion = await Conversion.init({
+		input,
+		output,
+		video: {
+			forceTranscode: true,
+		},
+		audio: {
+			forceTranscode: true,
+		},
+		trim: {
+			start: -10,
+			end: -5,
+		},
+	});
+	await conversion.execute();
+
+	using newInput = new Input({
+		source: new BufferSource(output.target.buffer!),
+		formats: ALL_FORMATS,
+	});
+	expect(await newInput.getPrimaryVideoTrack()).toBeNull(); // No video data
+	expect(await newInput.getPrimaryAudioTrack()).toBeNull(); // No audio data
+});
+
+test('Trim wholly before media data, copy', async () => {
+	using input = new Input({
+		source: new UrlSource('/demo.mp4'),
+		formats: ALL_FORMATS,
+	});
+	const output = new Output({
+		format: new Mp4OutputFormat(),
+		target: new BufferTarget(),
+	});
+
+	const conversion = await Conversion.init({
+		input,
+		output,
+		copy: { mode: 'forced' },
+		trim: {
+			start: -10,
+			end: -5,
+		},
+	});
+	await conversion.execute();
+
+	using newInput = new Input({
+		source: new BufferSource(output.target.buffer!),
+		formats: ALL_FORMATS,
+	});
+	expect(await newInput.getPrimaryVideoTrack()).toBeNull(); // No video data
+	expect(await newInput.getPrimaryAudioTrack()).toBeNull(); // No audio data
+});
+
+test('Trim wholly past media data, transcode', async () => {
+	using input = new Input({
+		source: new UrlSource('/demo.mp4'),
+		formats: ALL_FORMATS,
+	});
+	const output = new Output({
+		format: new Mp4OutputFormat(),
+		target: new BufferTarget(),
+	});
+
+	const conversion = await Conversion.init({
+		input,
+		output,
+		video: {
+			forceTranscode: true,
+		},
+		audio: {
+			forceTranscode: true,
+		},
+		trim: {
+			start: 10,
+			end: 15,
+		},
+	});
+	await conversion.execute();
+
+	using newInput = new Input({
+		source: new BufferSource(output.target.buffer!),
+		formats: ALL_FORMATS,
+	});
+	expect(await newInput.getPrimaryVideoTrack()).toBeNull(); // No video data
+	expect(await newInput.getPrimaryAudioTrack()).toBeNull(); // No audio data
+});
+
+test('Trim wholly past media data, copy', async () => {
+	using input = new Input({
+		source: new UrlSource('/demo.mp4'),
+		formats: ALL_FORMATS,
+	});
+	const output = new Output({
+		format: new Mp4OutputFormat(),
+		target: new BufferTarget(),
+	});
+
+	const conversion = await Conversion.init({
+		input,
+		output,
+		copy: { mode: 'forced' },
+		trim: {
+			start: 10,
+			end: 15,
+		},
+	});
+	await conversion.execute();
+
+	using newInput = new Input({
+		source: new BufferSource(output.target.buffer!),
+		formats: ALL_FORMATS,
+	});
+	// Don't test video yet; requires extensions to packet fetching logic. B-frames!!
+	// expect(await newInput.getPrimaryVideoTrack()).toBeNull(); // No video data
+	expect(await newInput.getPrimaryAudioTrack()).toBeNull(); // No audio data
+});