diff --git a/dev/convert.html b/dev/convert.html
index 94c7224..c49ed91 100644
--- a/dev/convert.html
+++ b/dev/convert.html
@@ -21,7 +21,7 @@
 			chunked: true,
 			chunkSize: 2**20
 		});
-		const outputFormat = new Metamuxer.Mp4OutputFormat();
+		const outputFormat = new Metamuxer.Mp4OutputFormat({ fastStart: 'fragmented' });
 
 		const button = document.createElement('button');
 		button.textContent = 'Cancel';
@@ -41,7 +41,7 @@
 				//numberOfChannels: 1,
 				//sampleRate: 4000
 				//discard: true
-				forceReencode: true,
+				//forceReencode: true,
 			},
 			/*
 			video: {
@@ -72,7 +72,7 @@
 				//width: 1280,
 				//discard: true,
 				//width: 640
-				forceReencode: true,
+				//forceReencode: true,
 				//rotate: 90
 				//width: 720 ?? 2160,
 				//height: 1280 ?? 3840,
@@ -85,7 +85,7 @@
 			},
 			trim: {
 				start: 0,
-				end: 40
+				end: 3
 			},
 		});
 		console.log(conversion);
diff --git a/dev/demux.html b/dev/demux.html
index 9e503ee..84863bc 100644
--- a/dev/demux.html
+++ b/dev/demux.html
@@ -15,10 +15,17 @@
 			formats: Metamuxer.ALL_FORMATS,
 			source
 		});
-
-		let timestamps = [];
+		
 		const videoTrack = await input.getPrimaryVideoTrack();
 
+		const sink = new Metamuxer.EncodedPacketSink(videoTrack);
+		for await (const packet of sink.packets()) {
+			if (packet.timestamp >= 2) break;
+			console.log(packet);
+		}
+
+		/*
+		let timestamps = [];
 		const packetSink = new Metamuxer.EncodedPacketSink(videoTrack);
 		for await (const packet of packetSink.packets()) {
 			if (packet.timestamp >= 1) {
@@ -71,22 +78,25 @@
 			7.253333333333333,
 			7.253333333333333,
 		];
+		*/
 
-		console.log(timestamps);
 		//return;
 		/*
+		console.log(timestamps);
 		const sink = new Metamuxer.EncodedPacketSink(videoTrack);
 		for await (const packet of sink.packets()) {
 			console.log(packet.sequenceNumber);
 		}
 		*/
 
+		/*
 		const sink = new Metamuxer.VideoSampleSink(videoTrack);
 		for await (const sample of sink.samplesAtTimestamps(timestamps)) {
 			console.log("wee", sample.timestamp);
 			sample.close();
 		}
 		console.log("don")
+		*/
 
 		/*
 		const sink = new Metamuxer.CanvasSink(videoTrack, { width: 320 });
@@ -100,6 +110,7 @@
 		
 		
 
+		/*
 		for (let i = 0; i < 40; i++) {
 			const count = Math.floor(50 * Math.random());
 			const timestamps = Array.from({ length: count }, () => Math.random() * 20).sort((a, b) => a-b);
@@ -112,6 +123,7 @@
 		}
 
 		console.log("DONE!")
+		*/
 		
 		
 			
diff --git a/src/isobmff/isobmff-muxer.ts b/src/isobmff/isobmff-muxer.ts
index 27537e9..234f349 100644
--- a/src/isobmff/isobmff-muxer.ts
+++ b/src/isobmff/isobmff-muxer.ts
@@ -37,6 +37,7 @@ export type Sample = {
 };
 
 type Chunk = {
+	/** The lowest presentation timestamp in this chunk */
 	startTimestamp: number;
 	samples: Sample[];
 	offset: number | null;
@@ -592,7 +593,7 @@ export class IsobmffMuxer extends Muxer {
 		return sample;
 	}
 
-	private processTimestamps(trackData: IsobmffTrackData) {
+	private processTimestamps(trackData: IsobmffTrackData, nextSample?: Sample) {
 		if (trackData.timestampProcessingQueue.length === 0) {
 			return;
 		}
@@ -725,9 +726,29 @@ export class IsobmffMuxer extends Muxer {
 		}
 
 		trackData.timestampProcessingQueue.length = 0;
+
+		assert(trackData.lastSample);
+		assert(trackData.lastTimescaleUnits !== null);
+
+		if (nextSample !== undefined && trackData.lastSample.timescaleUnitsToNextSample === 0) {
+			assert(nextSample.type === 'key');
+
+			// Given the next sample, we can make a guess about the duration of the last sample. This avoids having
+			// the last sample's duration in each fragment be "0" for fragmented files. The guess we make here is
+			// actually correct most of the time, since typically, no delta frame with a lower timestamp follows the key
+			// frame (although it can happen).
+			const timescaleUnits = intoTimescale(nextSample.timestamp, trackData.timescale, false);
+			const delta = Math.round(timescaleUnits - trackData.lastTimescaleUnits);
+			trackData.lastSample.timescaleUnitsToNextSample = delta;
+		}
 	}
 
 	private async registerSample(trackData: IsobmffTrackData, sample: Sample) {
+		if (sample.type === 'key') {
+			this.processTimestamps(trackData, sample);
+		}
+		trackData.timestampProcessingQueue.push(sample);
+
 		if (this.isFragmented) {
 			trackData.sampleQueue.push(sample);
 			await this.interleaveSamples();
@@ -737,10 +758,6 @@ export class IsobmffMuxer extends Muxer {
 	}
 
 	private async addSampleToTrack(trackData: IsobmffTrackData, sample: Sample) {
-		if (sample.type === 'key') {
-			this.processTimestamps(trackData);
-		}
-
 		if (!this.isFragmented) {
 			trackData.samples.push(sample);
 		}
@@ -749,6 +766,13 @@ export class IsobmffMuxer extends Muxer {
 		if (!trackData.currentChunk) {
 			beginNewChunk = true;
 		} else {
+			// Timestamp don't need to be monotonic (think B-frames), so we may need to update the start timestamp of
+			// the chunk
+			trackData.currentChunk.startTimestamp = Math.min(
+				trackData.currentChunk.startTimestamp,
+				sample.timestamp,
+			);
+
 			const currentChunkDuration = sample.timestamp - trackData.currentChunk.startTimestamp;
 
 			if (this.isFragmented) {
@@ -795,7 +819,6 @@ export class IsobmffMuxer extends Muxer {
 
 		assert(trackData.currentChunk);
 		trackData.currentChunk.samples.push(sample);
-		trackData.timestampProcessingQueue.push(sample);
 
 		if (this.isFragmented) {
 			this.maxWrittenTimestamp = Math.max(this.maxWrittenTimestamp, sample.timestamp);
@@ -1003,6 +1026,11 @@ export class IsobmffMuxer extends Muxer {
 
 		if (this.isFragmented) {
 			await this.interleaveSamples(true);
+
+			for (const trackData of this.trackDatas) {
+				this.processTimestamps(trackData);
+			}
+
 			await this.finalizeFragment(false); // Don't flush the last fragment as we will flush it with the mfra box
 		} else {
 			for (const trackData of this.trackDatas) {