diff --git a/build/types/core b/build/types/core
index c6d8a0c3dc..b14a78152c 100644
--- a/build/types/core
+++ b/build/types/core
@@ -77,6 +77,7 @@
 +../../lib/util/map_utils.js
 +../../lib/util/media_ready_state_utils.js
 +../../lib/util/mime_utils.js
++../../lib/util/mp4_box_parsers.js
 +../../lib/util/mp4_parser.js
 +../../lib/util/multi_map.js
 +../../lib/util/networking.js
diff --git a/externs/shaka/mp4_boxes.js b/externs/shaka/mp4_boxes.js
new file mode 100644
index 0000000000..3016cfebe6
--- /dev/null
+++ b/externs/shaka/mp4_boxes.js
@@ -0,0 +1,95 @@
+/*! @license
+ * Shaka Player
+ * Copyright 2016 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+
+/**
+ * @externs
+ */
+
+/**
+ * @typedef {{
+ *    defaultSampleDuration: ?number
+ * }}
+ *
+ * @property {?number} defaultSampleDuration
+ *   If specified via flags, this overrides the default sample
+ *   duration in the Track Extends Box for this fragment
+ *
+ * @exportDoc
+ */
+shaka.extern.ParsedTFHDBox;
+
+/**
+ * @typedef {{
+ *    baseMediaDecodeTime: !number
+ * }}
+ *
+ * @property {!number} baseMediaDecodeTime
+ *  As per the spec: the absolute decode time, measured on the media
+ *  timeline, of the first sample in decode order in the track fragment
+ *
+ * @exportDoc
+ */
+shaka.extern.ParsedTFDTBox;
+
+/**
+ * @typedef {{
+ *    timescale: !number
+ * }}
+ *
+ * @property {!number} timescale
+ *  As per the spec: an integer that specifies the time‐scale for this media;
+ * this is the number of time units that pass in one second
+ *
+ * @exportDoc
+ */
+shaka.extern.ParsedMDHDBox;
+
+/**
+ * @typedef {{
+ *    defaultSampleDuration: !number
+ * }}
+ *
+ * @property {!number} defaultSampleDuration
+ *  The default sample duration to be used in track fragments
+ *
+ * @exportDoc
+ */
+shaka.extern.ParsedTREXBox;
+
+/**
+ * @typedef {{
+ *    sampleCount: !number,
+ *    sampleData: !Array.<shaka.extern.TRUNSample>
+ * }}
+ *
+ * @property {!number} sampleCount
+ *  As per the spec: the number of samples being added in this run;
+ * @property {!Array.<shaka.extern.TRUNSample>} sampleData
+ *  An array of size <sampleCount> containing data for each sample
+ *
+ * @exportDoc
+ */
+shaka.extern.ParsedTRUNBox;
+
+/**
+ * @typedef {{
+ *    sampleDuration: ?number,
+ *    sampleSize: ?number,
+ *    sampleCompositionTimeOffset: ?number
+ *  }}
+ *
+ * @property {?number} sampleDuration
+ *    The length of the sample in timescale units.
+ * @property {?number} sampleSize
+ *    The size of the sample in bytes.
+ * @property {?number} sampleCompositionTimeOffset
+ *    The time since the start of the sample in timescale units. Time
+ *    offset is based of the start of the sample. If this value is
+ *    missing, the accumated durations preceeding this time sample will
+ *    be used to create the start time.
+ */
+shaka.extern.TRUNSample;
diff --git a/lib/hls/hls_parser.js b/lib/hls/hls_parser.js
index 040b4fef92..7cbfe05625 100644
--- a/lib/hls/hls_parser.js
+++ b/lib/hls/hls_parser.js
@@ -32,6 +32,7 @@ goog.require('shaka.util.LanguageUtils');
 goog.require('shaka.util.ManifestParserUtils');
 goog.require('shaka.util.MimeUtils');
 goog.require('shaka.util.Mp4Parser');
+goog.require('shaka.util.Mp4BoxParsers');
 goog.require('shaka.util.Networking');
 goog.require('shaka.util.OperationManager');
 goog.require('shaka.util.Timer');
@@ -1864,12 +1865,10 @@ shaka.hls.HlsParser = class {
               box.version == 0 || box.version == 1,
               'MDHD version can only be 0 or 1');
 
-          // Skip "creation_time" and "modification_time".
-          // They are 4 bytes each if the mdhd box is version 0, 8 bytes each
-          // if it is version 1.
-          box.reader.skip(box.version == 0 ? 8 : 16);
+          const parsedMDHDBox = shaka.util.Mp4BoxParsers.parseMDHD(
+              box.reader, box.version);
 
-          timescale = box.reader.readUint32();
+          timescale = parsedMDHDBox.timescale;
           box.parser.stop();
         }).parse(initData, /* partialOkay= */ true);
 
@@ -1891,9 +1890,10 @@ shaka.hls.HlsParser = class {
           goog.asserts.assert(
               box.version == 0 || box.version == 1,
               'TFDT version can only be 0 or 1');
-          const baseTime = (box.version == 0) ?
-          box.reader.readUint32() :
-          box.reader.readUint64();
+
+          const parsedTFDTBox = shaka.util.Mp4BoxParsers.parseTFDT(
+              box.reader, box.version);
+          const baseTime = parsedTFDTBox.baseMediaDecodeTime;
           startTime = baseTime / timescale;
           parsedMedia = true;
           box.parser.stop();
diff --git a/lib/text/mp4_vtt_parser.js b/lib/text/mp4_vtt_parser.js
index 8d1e8c4805..46debbdbe1 100644
--- a/lib/text/mp4_vtt_parser.js
+++ b/lib/text/mp4_vtt_parser.js
@@ -14,8 +14,8 @@ goog.require('shaka.text.VttTextParser');
 goog.require('shaka.util.DataViewReader');
 goog.require('shaka.util.Error');
 goog.require('shaka.util.Functional');
-goog.require('shaka.util.Iterables');
 goog.require('shaka.util.Mp4Parser');
+goog.require('shaka.util.Mp4BoxParsers');
 goog.require('shaka.util.StringUtils');
 goog.require('shaka.util.TextParser');
 
@@ -52,18 +52,10 @@ shaka.text.Mp4VttParser = class {
           goog.asserts.assert(
               box.version == 0 || box.version == 1,
               'MDHD version can only be 0 or 1');
-          if (box.version == 0) {
-            box.reader.skip(4); // Skip "creation_time".
-            box.reader.skip(4); // Skip "modification_time".
-            this.timescale_ = box.reader.readUint32();
-            box.reader.skip(4); // Skip "duration".
-          } else {
-            box.reader.skip(8); // Skip "creation_time".
-            box.reader.skip(8); // Skip "modification_time".
-            this.timescale_ = box.reader.readUint32();
-            box.reader.skip(8); // Skip "duration".
-          }
-          box.reader.skip(4); // Skip "pad", "language", and "pre-defined".
+
+          const parsedMDHDBox = shaka.util.Mp4BoxParsers.parseMDHD(
+              box.reader, box.version);
+          this.timescale_ = parsedMDHDBox.timescale;
         })
         .box('minf', Mp4Parser.children)
         .box('stbl', Mp4Parser.children)
@@ -106,11 +98,10 @@ shaka.text.Mp4VttParser = class {
           shaka.util.Error.Code.INVALID_MP4_VTT);
     }
 
-    const Mp4VttParser = shaka.text.Mp4VttParser;
     const Mp4Parser = shaka.util.Mp4Parser;
 
     let baseTime = 0;
-    /** @type {!Array.<shaka.text.Mp4VttParser.TimeSegment>} */
+    /** @type {!Array.<shaka.extern.TRUNSample>} */
     let presentations = [];
     /** @type {!Uint8Array} */
     let rawPayload;
@@ -130,14 +121,18 @@ shaka.text.Mp4VttParser = class {
           goog.asserts.assert(
               box.version == 0 || box.version == 1,
               'TFDT version can only be 0 or 1');
-          baseTime = (box.version == 0) ? box.reader.readUint32() :
-                                          box.reader.readUint64();
+
+          const parsedTFDTBox = shaka.util.Mp4BoxParsers.parseTFDT(
+              box.reader, box.version);
+          baseTime = parsedTFDTBox.baseMediaDecodeTime;
         })
         .fullBox('tfhd', (box) => {
           goog.asserts.assert(
               box.flags != null,
               'A TFHD box should have a valid flags value');
-          defaultDuration = Mp4VttParser.parseTFHD_(box.flags, box.reader);
+          const parsedTFHDBox = shaka.util.Mp4BoxParsers.parseTFHD(
+              box.reader, box.flags);
+          defaultDuration = parsedTFHDBox.defaultSampleDuration;
         })
         .fullBox('trun', (box) => {
           sawTRUN = true;
@@ -147,8 +142,10 @@ shaka.text.Mp4VttParser = class {
           goog.asserts.assert(
               box.flags != null,
               'A TRUN box should have a valid flags value');
-          presentations =
-              Mp4VttParser.parseTRUN_(box.version, box.flags, box.reader);
+
+          const parsedTRUNBox = shaka.util.Mp4BoxParsers.parseTRUN(
+              box.reader, box.version, box.flags);
+          presentations = parsedTRUNBox.sampleData;
         })
         .box('mdat', Mp4Parser.allData((data) => {
           goog.asserts.assert(
@@ -176,9 +173,9 @@ shaka.text.Mp4VttParser = class {
     for (const presentation of presentations) {
       // If one presentation corresponds to multiple payloads, it is assumed
       // that all of those payloads have the same start time and duration.
-      const duration = presentation.duration || defaultDuration;
-      const startTime = presentation.timeOffset ?
-                      baseTime + presentation.timeOffset :
+      const duration = presentation.sampleDuration || defaultDuration;
+      const startTime = presentation.sampleCompositionTimeOffset ?
+                      baseTime + presentation.sampleCompositionTimeOffset :
                       currentTime;
       currentTime = startTime + (duration || 0);
 
@@ -244,94 +241,6 @@ shaka.text.Mp4VttParser = class {
       cues.filter(shaka.util.Functional.isNotNull));
   }
 
-  /**
-   * @param {number} flags
-   * @param {!shaka.util.DataViewReader} reader
-   * @return {?number} The default_sample_duration field, if present.
-   * @private
-   */
-  static parseTFHD_(flags, reader) {
-    // Skip "track_ID".
-    reader.skip(4);
-
-    // Skip "base_data_offset" if present.
-    if (flags & 0x000001) {
-      reader.skip(8);
-    }
-
-    // Skip "sample_description_index" if present.
-    if (flags & 0x000002) {
-      reader.skip(4);
-    }
-
-    // Read and return "default_sample_duration" if present.
-    if (flags & 0x000008) {
-      return reader.readUint32();
-    }
-
-    // There is no "default_sample_duration".
-    return null;
-  }
-
-  /**
-   * @param {number} version
-   * @param {number} flags
-   * @param {!shaka.util.DataViewReader} reader
-   * @return {!Array.<shaka.text.Mp4VttParser.TimeSegment>}
-   * @private
-   */
-  static parseTRUN_(version, flags, reader) {
-    const sampleCount = reader.readUint32();
-
-    // Skip "data_offset" if present.
-    if (flags & 0x000001) {
-      reader.skip(4);
-    }
-
-    // Skip "first_sample_flags" if present.
-    if (flags & 0x000004) {
-      reader.skip(4);
-    }
-
-    const samples = [];
-
-    for (const _ of shaka.util.Iterables.range(sampleCount)) {
-      shaka.util.Functional.ignored(_);
-      /** @type {shaka.text.Mp4VttParser.TimeSegment} */
-      const sample = {
-        duration: null,
-        sampleSize: null,
-        timeOffset: null,
-      };
-
-      // Read "sample duration" if present.
-      if (flags & 0x000100) {
-        sample.duration = reader.readUint32();
-      }
-
-      // Read "sample_size" if present.
-      if (flags & 0x000200) {
-        sample.sampleSize = reader.readUint32();
-      }
-
-      // Skip "sample_flags" if present.
-      if (flags & 0x000400) {
-        reader.skip(4);
-      }
-
-      // Read "sample_time_offset" if present.
-      if (flags & 0x000800) {
-        sample.timeOffset = version == 0 ?
-            reader.readUint32() :
-            reader.readInt32();
-      }
-
-      samples.push(sample);
-    }
-
-    return samples;
-  }
-
   /**
    * Parses a vttc box into a cue.
    *
@@ -407,24 +316,5 @@ shaka.text.Mp4VttParser = class {
   }
 };
 
-/**
- * @typedef {{
- *    duration: ?number,
- *    sampleSize: ?number,
- *    timeOffset: ?number
- *  }}
- *
- * @property {?number} duration
- *    The length of the segment in timescale units.
- * @property {?number} sampleSize
- *    The size of the segment in bytes.
- * @property {?number} timeOffset
- *    The time since the start of the segment in timescale units. Time
- *    offset is based of the start of the segment. If this value is
- *    missing, the accumated durations preceeding this time segment will
- *    be used to create the start time.
- */
-shaka.text.Mp4VttParser.TimeSegment;
-
 shaka.text.TextEngine.registerParser(
     'application/mp4; codecs="wvtt"', () => new shaka.text.Mp4VttParser());
diff --git a/lib/util/mp4_box_parsers.js b/lib/util/mp4_box_parsers.js
new file mode 100644
index 0000000000..0eba371920
--- /dev/null
+++ b/lib/util/mp4_box_parsers.js
@@ -0,0 +1,171 @@
+/*! @license
+ * Shaka Player
+ * Copyright 2016 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+goog.provide('shaka.util.Mp4BoxParsers');
+
+goog.require('shaka.util.DataViewReader');
+goog.require('shaka.util.Iterables');
+
+shaka.util.Mp4BoxParsers = class {
+  /**
+   * Parses a TFHD Box.
+   * @param {!shaka.util.DataViewReader} reader
+   * @param {!number} flags
+   * @return {!shaka.extern.ParsedTFHDBox}
+   */
+  static parseTFHD(reader, flags) {
+    let defaultSampleDuration = null;
+
+    reader.skip(4); // Skip "track_ID"
+
+    // Skip "base_data_offset" if present.
+    if (flags & 0x000001) {
+      reader.skip(8);
+    }
+
+    // Skip "sample_description_index" if present.
+    if (flags & 0x000002) {
+      reader.skip(4);
+    }
+
+    // Read "default_sample_duration" if present.
+    if (flags & 0x000008) {
+      defaultSampleDuration = reader.readUint32();
+    }
+
+    /** @type {!shaka.extern.ParsedTFHDBox} */
+    const parsedTFHD = {
+      defaultSampleDuration: defaultSampleDuration,
+    };
+
+    return parsedTFHD;
+  }
+
+  /**
+   * Parses a TFDT Box.
+   * @param {!shaka.util.DataViewReader} reader
+   * @param {!number} version
+   * @return {!shaka.extern.ParsedTFDTBox}
+   */
+  static parseTFDT(reader, version) {
+    const baseMediaDecodeTime = version == 1 ?
+        reader.readUint64() : reader.readUint32();
+
+    /** @type {!shaka.extern.ParsedTFDTBox} */
+    const parsedTFDT = {
+      baseMediaDecodeTime: baseMediaDecodeTime,
+    };
+
+    return parsedTFDT;
+  }
+
+  /**
+   * Parses a MDHD Box.
+   * @param {!shaka.util.DataViewReader} reader
+   * @param {!number} version
+   * @return {!shaka.extern.ParsedMDHDBox}
+   */
+  static parseMDHD(reader, version) {
+    if (version == 1) {
+      reader.skip(8); // Skip "creation_time"
+      reader.skip(8); // Skip "modification_time"
+    } else {
+      reader.skip(4); // Skip "creation_time"
+      reader.skip(4); // Skip "modification_time"
+    }
+
+    const timescale = reader.readUint32();
+
+    /** @type {!shaka.extern.ParsedMDHDBox} */
+    const parsedMDHD = {
+      timescale: timescale,
+    };
+
+    return parsedMDHD;
+  }
+
+  /**
+   * Parses a TREX Box.
+   * @param {!shaka.util.DataViewReader} reader
+   * @return {!shaka.extern.ParsedTREXBox}
+   */
+  static parseTREX(reader) {
+    reader.skip(4); // Skip "track_ID"
+    reader.skip(4); // Skip "default_sample_description_index"
+    const defaultSampleDuration = reader.readUint32();
+
+    /** @type {!shaka.extern.ParsedTREXBox} */
+    const parsedTREX = {
+      defaultSampleDuration: defaultSampleDuration,
+    };
+
+    return parsedTREX;
+  }
+
+  /**
+   * Parses a TRUN Box.
+   * @param {!shaka.util.DataViewReader} reader
+   * @param {!number} version
+   * @param {!number} flags
+   * @return {!shaka.extern.ParsedTRUNBox}
+   */
+  static parseTRUN(reader, version, flags) {
+    const sampleCount = reader.readUint32();
+    const sampleData = [];
+
+    // Skip "data_offset" if present.
+    if (flags & 0x000001) {
+      reader.skip(4);
+    }
+
+    // Skip "first_sample_flags" if present.
+    if (flags & 0x000004) {
+      reader.skip(4);
+    }
+
+    for (const _ of shaka.util.Iterables.range(sampleCount)) {
+      shaka.util.Functional.ignored(_);
+      /** @type {shaka.extern.TRUNSample} */
+      const sample = {
+        sampleDuration: null,
+        sampleSize: null,
+        sampleCompositionTimeOffset: null,
+      };
+
+      // Read "sample duration" if present.
+      if (flags & 0x000100) {
+        sample.sampleDuration = reader.readUint32();
+      }
+
+      // Read "sample_size" if present.
+      if (flags & 0x000200) {
+        sample.sampleSize = reader.readUint32();
+      }
+
+      // Skip "sample_flags" if present.
+      if (flags & 0x000400) {
+        reader.skip(4);
+      }
+
+      // Read "sample_time_offset" if present.
+      if (flags & 0x000800) {
+        sample.sampleCompositionTimeOffset = version == 0 ?
+              reader.readUint32() :
+              reader.readInt32();
+      }
+
+      sampleData.push(sample);
+    }
+
+    /** @type {!shaka.extern.ParsedTRUNBox} */
+    const parsedTRUN = {
+      sampleCount: sampleCount,
+      sampleData: sampleData,
+    };
+
+    return parsedTRUN;
+  }
+};
diff --git a/test/media/transmuxer_integration.js b/test/media/transmuxer_integration.js
index 10c0c7190d..fb7af1b836 100644
--- a/test/media/transmuxer_integration.js
+++ b/test/media/transmuxer_integration.js
@@ -154,9 +154,9 @@ describe('Transmuxer', () => {
             goog.asserts.assert(
                 box.version == 0 || box.version == 1,
                 'TFDT version can only be 0 or 1');
-            mp4Timestamp = (box.version == 0) ?
-                box.reader.readUint32() :
-                box.reader.readUint64();
+            const parsedTFDTBox = shaka.util.Mp4BoxParsers.parseTFDT(
+                box.reader, box.version);
+            mp4Timestamp = parsedTFDTBox.baseMediaDecodeTime;
             parsed = true;
           })
           .parse(transmuxedData.data);