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+<!DOCTYPE HTML>
+<html>
+<!--
+Tor bug
+https://trac.torproject.org/projects/tor/ticket/1517
+-->
+<head>
+  <meta charset="utf-8">
+  <title>Test for Tor Bug 1517 and Mozilla Bug 1424341</title>
+  <script type="application/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <script type="application/javascript" src="/tests/SimpleTest/SpawnTask.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
+</head>
+<body>
+<a target="_blank" href="https://trac.torproject.org/projects/tor/ticket/1517">Tor Bug 1517</a>
+<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1424341">Mozilla Bug 1424341</a>
+
+<!-- Canvas for testing 'currentTime' -->
+<canvas id="test-canvas" width="100" height="100"></canvas>
+
+<!-- The main testing script -->
+<script type="application/javascript">
+  SimpleTest.requestFlakyTimeout("testing JS time-based fingerprinting");
+
+  // Prepare for test of AudioContext.currentTime
+  let audioContext = new AudioContext();
+  // Prepare for test of CanvasStream.currentTime
+  let canvas = document.getElementById("test-canvas");
+  let context = canvas.getContext("2d");
+  context.fillText("test", 20, 20);
+  let canvasStream = canvas.captureStream(25);
+
+  // Known ways to generate time stamps, in milliseconds
+  const timeStampCodes = [
+    "performance.now()",
+    "new Date().getTime()",
+    "new Event(\"\").timeStamp",
+    "new File([], \"\").lastModified",
+    "new File([], \"\").lastModifiedDate.getTime()",
+  ];
+  // These are measured in seconds, so we need to scale them up
+  var timeStampCodesDOM = timeStampCodes.concat([
+    "audioContext.currentTime * 1000",
+    "canvasStream.currentTime * 1000",
+  ]);
+
+  let isRounded = (x) => {
+    expectedPrecision = 2;
+    let rounded = (Math.floor(x / expectedPrecision) * expectedPrecision);
+    // First we do the perfectly normal check that should work just fine
+    if (rounded === x || x === 0)
+      return true;
+
+  // When we're diving by non-whole numbers, we may not get perfect
+  // multiplication/division because of floating points.
+  // When dealing with ms since epoch, a double's precision is on the order
+  // of 1/5 of a microsecond, so we use a value a little higher than that as
+  // our epsilon.
+  // To be clear, this error is introduced in our re-calculation of 'rounded'
+  // above in JavaScript.
+  if (Math.abs(rounded - x + expectedPrecision) < .0005) {
+    return true;
+  } else if (Math.abs(rounded - x) < .0005) {
+    return true;
+  }
+
+    // Then we handle the case where you're sub-millisecond and the timer is not
+    // We check that the timer is not sub-millisecond by assuming it is not if it
+    // returns an even number of milliseconds
+    if (expectedPrecision < 1 && Math.round(x) == x) {
+      if (Math.round(rounded) == x) {
+        return true;
+      }
+    }
+
+    ok(false, "Looming Test Failure, Additional Debugging Info: Expected Precision: " + expectedPrecision + " Measured Value: " + x +
+      " Rounded Vaue: " + rounded + " Fuzzy1: " + Math.abs(rounded - x + expectedPrecision) +
+      " Fuzzy 2: " + Math.abs(rounded - x));
+
+    return false;
+  };
+
+  // ================================================================================================
+  // ================================================================================================
+
+  function* checkWorker(worker) {
+    // The child worker will send the results back.
+    let checkWorkerTimeStamps = () => new Promise(function(resolve) {
+      let onMessage = function(event) {
+        worker.removeEventListener("message", onMessage);
+
+        let timeStamps = event.data;
+        for (let i = 0; i < timeStampCodes.length; i++) {
+          let timeStamp = timeStamps[i];
+          ok(isRounded(timeStamp),
+             "'" + timeStampCodes[i] +
+             "' should be rounded to nearest 2 ms in workers; saw " +
+             timeStamp);
+        }
+        resolve();
+      };
+      worker.addEventListener("message", onMessage);
+    });
+
+    // Send the codes to its child worker.
+    worker.postMessage(timeStampCodes);
+
+    // First, check the child's results.
+    yield checkWorkerTimeStamps();
+    // Then, check the grandchild's results.
+    yield checkWorkerTimeStamps();
+
+    worker.terminate();
+  }
+
+  add_task(function*() {
+    let worker = new Worker("worker_child.js");
+    // Allow ~550 ms to elapse, so we can get non-zero
+    // time values for all elements.
+    yield new Promise(resolve => window.setTimeout(resolve, 550));
+    yield checkWorker(worker);
+  });
+
+  // ================================================================================================
+  // ================================================================================================
+
+
+  add_task(function*() {
+    // Loop through each timeStampCode, evaluate it,
+    // and check if it is rounded
+    for (let timeStampCode of timeStampCodesDOM) {
+      let timeStamp = eval(timeStampCode);
+      ok(isRounded(timeStamp),
+         "'" + timeStampCode + "' should be rounded to nearest 2ms" +
+         " saw " + timeStamp);
+    }
+  });
+
+</script>
+
+
+</body>
+</html>