This specification defines an API that provides the current time in sub-millisecond resolution and such that it is not subject to system clock skew or adjustments.

High Resolution Time Level 3 replaces the second version of High Resolution Time [[HR-TIME-2]] and includes:

Introduction

The ECMAScript Language specification [[ECMA-262]] defines the Date object as a time value representing time in milliseconds since 01 January, 1970 UTC. For most purposes, this definition of time is sufficient as these values represent time to millisecond precision for any instant that is within approximately 285,616 years from 01 January, 1970 UTC. The [DOMTimeStamp] is defined similarly [[WebIDL]].

In practice, these definitions of time are subject to both clock skew and adjustment of the system clock. The value of time may not always be monotonically increasing and subsequent values may either decrease or remain the same.

For example, the following script may record a positive number, negative number, or zero for computed `duration`:

var mark_start = Date.now();
doTask(); // Some task
var duration = Date.now() - mark_start;

For certain tasks this definition of time may not be sufficient as it does not allow for sub-millisecond resolution and is subject to system clock skew. For example,

This specification does not propose changing the behavior of Date.now() [[ECMA-262]] as it is genuinely useful in determining the current value of the calendar time and has a long history of usage. The DOMHighResTimeStamp type, performance.now method, and performance.timeOrigin attributes of the Performance interface resolve above issues by providing monotonically increasing time values with sub-millisecond resolution.

Examples

A developer may wish to construct a timeline of their entire application, including events from [dedicated][dedicatedworker] or [shared workers][sharedworker], which have different time origin's. To display such events on the same timeline, the application can translate the DOMHighResTimeStamp's with the help of the performance.timeOrigin attribute.

// ---- worker.js -----------------------------
// Shared worker script
onconnect = function(e) {
  var port = e.ports[0];
  port.onmessage = function(e) {
    // Time execution in worker
    var task_start = performance.now();
    result = runSomeWorkerTask();
    var task_end = performance.now();

    // Send results and epoch-relative timestamps to another context
    port.postMessage({
       'task': 'Some worker task',
       'start_time': task_start + performance.timeOrigin,
       'end_time': task_end + performance.timeOrigin,
       'result': result
    });
  }
}

// ---- application.js ------------------------
// Timing tasks in the document
var task_start = performance.now();
runSomeApplicationTask();
var task_end = performance.now();

// developer provided method to upload runtime performance data
reportEventToAnalytics({
  'task': 'Some document task',
  'start_time': task_start,
  'duration': task_end - task_start
});

// Translating worker timestamps into document's time origin
var worker = new SharedWorker('worker.js');
worker.port.onmessage = function (event) {
  var msg = event.data;

  // translate epoch-relative timestamps into document's time origin
  msg.start_time = msg.start_time - performance.timeOrigin;
  msg.end_time = msg.end_time - performance.timeOrigin;

  reportEventToAnalytics(msg);
}

Some conformance requirements are phrased as requirements on attributes, methods or objects. Such requirements are to be interpreted as requirements on user agents.

The IDL fragments in this specification must be interpreted as required for conforming IDL fragments, as described in the Web IDL specification. [[!WebIDL]]

Time Origin

The time origin is the time value from which time is measured:

The time origin timestamp is the high resolution time value at which time origin is zero. To obtain the time origin timestamp given a [global object] (global):

The time origin timestamp and the value returned by `Date.now()` executed at "zero time" can differ because the former is recorded with respect to a global monotonic clock that is not subject to system and user clock adjustments, clock skew, and so on—see .

The DOMHighResTimeStamp Type

The DOMHighResTimeStamp type is used to store a time value in milliseconds, measured relative from the time origin, global monotonic clock, or a time value that represents a duration between two DOMHighResTimeStamp's.

typedef double DOMHighResTimeStamp;

A DOMHighResTimeStamp SHOULD represent a time in milliseconds accurate to 5 microseconds - see .

If the User Agent is unable to provide a time value accurate to 5 microseconds due to hardware or software constraints, the User Agent can represent a DOMHighResTimeStamp as a time in milliseconds accurate to a millisecond.

The Performance interface

[Exposed=(Window,Worker)]
interface Performance : EventTarget {
    DOMHighResTimeStamp now ();
    readonly attribute DOMHighResTimeStamp timeOrigin;
    serializer = {attribute};
};

The now() method MUST return a DOMHighResTimeStamp representing the high resolution time from the time origin to the occurrence of the call to the Performance.now method.

The timeOrigin attribute MUST return a DOMHighResTimeStamp representing the high resolution time of the time origin timestamp for the [relevant global object] of the Performance object.

The serializer provide a way for the Performance object to be converted into a serialized form.

The performance attribute

The performance attribute on the WindowOrWorkerGlobalScope allows access to performance related attributes and methods from the [global object].

partial interface WindowOrWorkerGlobalScope {
    [Replaceable]
    readonly    attribute Performance performance;
};

Monotonic Clock

The time values returned when calling the Performance.now method on Performance objects with the same time origin MUST use the same monotonic clock that is monotonically increasing and not subject to system clock adjustments or system clock skew. The difference between any two chronologically recorded time values returned from the Performance.now method MUST never be negative if the two time values have the same time origin.

The time values returned when getting performance.timeOrigin MUST use the same global monotonic clock that is shared by time origin's, is monotonically increasing and not subject to system clock adjustments or system clock skew, and whose reference point is the Unix time—see .

The user agent can reset its global monotonic clock across browser restarts, or whenever starting an isolated browsing session—e.g. incognito or similar browsing mode. As a result, developers should not use global timestamps as absolute time that holds its monotonic properties across all past, present, and future contexts; in practice, the monotonic properties only apply for contexts that can reach other by exchanging messages via one of the provided messaging mechanisms - e.g. `postMessage`, `BroadcastChannel`, etc.

Privacy and Security

Clock resolution

Access to accurate timing information, both for measurement and scheduling purposes, is a common requirement for many applications. For example, coordinating animations, sound, and other activity on the page requires access to high-resolution time to provide a good user experience. Similarly, measurement enables developers to track the performance of critical code components, detect regressions, and so on.

However, access to the same accurate timing information can sometimes be also used for malicious purposes by an attacker to guess and infer data that they can't see or access otherwise. For example, cache attacks and statistical fingerprinting is a privacy and security concern where a malicious web site may use high resolution timing data of various browser or application-initiated operations to differentiate between subset of users, and in some cases identify a particular user - see [[CACHE-ATTACKS]].

This specification defines an API that provides sub-millisecond time resolution, which is more accurate than the previously available millisecond resolution exposed by [DOMTimeStamp]. However, even without this new API an attacker may be able to obtain high-resolution estimates through repeat execution and statistical analysis. To ensure that the new API does not significantly improve the accuracy or speed of such attacks, the recommended minimum resolution of the Performance interface should be set to 5 microseconds.

Mitigating such timing side-channel attacks entirely is practically not possible: either all operations would have to execute in a time that does not vary based on the value of any confidential information, or, the application would need to be isolated from any time-related primitives (clock, timers, counters, etc). Neither is practical due to the associated complexity for the browser and application developers and the associated negative effects on performance and responsiveness of applications.

Clock drift

This specification also defines an API that provides sub-millisecond time resolution of the zero time of the time origin, which requires and exposes a global monotonic clock to the application, and that must be shared across all the browser contexts. The global monotonic clock does not need to be tied to physical time, but is recommended to be set with respect to the [Unix time] to avoid exposing new fingerprint entropy about the user—e.g. this time can already be easily obtained by the application, whereas exposing a new logical clock provides new information.

However, even with above mechanism in place, the global monotonic clock may provide additional [clock drift] resolution. Today, the application can timestamp the time-of-day and monotonic time values (via `Date.now()` and `performance.now()`) at multiple points within the same context and observe drift between them—e.g. due to automatic or user clock adjustments. With the performance.timeOrigin attribute, the attacker can also compare the time at which time origin is zero, as reported by the global monotonic clock, against the current time-of-day estimate of when it is zero (i.e. difference between `Date.now()-performance.now()` and `performance.timeOrigin`) and potentially observe clock drift between these clocks over a longer time period.

In practice, the same time drift can be observed by an application across multiple navigations: the application can record logical time in each context and use a client or server time synchronization mechanism to infer changes in the user's clock. Similarly, lower-layer mechanisms such as TCP timestamps may reveal same high-resolution information to the server without the need for multiple visits. As such, the information provided by this API should not expose any significant or previously not available entropy about the user.

Acknowledgments

Thanks to Arvind Jain, Angelos D. Keromytis, Boris Zbarsky, Jason Weber, Karen Anderson, Nat Duca, Philippe Le Hegaret, Ryosuke Niwa, Simha Sethumadhavan, Todd Reifsteck, Tony Gentilcore, Vasileios P. Kemerlis, Yoav Weiss, and Yossef Oren for their contributions to this work.

[DOMTimeStamp]: https://www.w3.org/TR/WebIDL/#common-DOMTimeStamp [dedicatedworker]: https://html.spec.whatwg.org/multipage/workers.html#worker [sharedworker]: https://html.spec.whatwg.org/multipage/workers.html#sharedworker [official moment of creation]: https://html.spec.whatwg.org/multipage/workers.html#worker-processing-model [WorkerGlobalScope]: https://html.spec.whatwg.org/multipage/workers.html#workerglobalscope [global object]: https://www.w3.org/TR/html51/webappapis.html#global-object [relevant global object]: https://www.w3.org/TR/html51/webappapis.html#relevant [clock drift]: https://en.wikipedia.org/wiki/Clock_drift [Unix time]: https://en.wikipedia.org/wiki/Unix_time