High Resolution Time

Abstract

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

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.

This document was published by the Web Performance Working Group as a Working Draft. This document is intended to become a W3C Recommendation.

GitHub Issues are preferred for discussion of this specification.

Publication as a Working Draft does not imply endorsement by the W3C Membership.

This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 15 September 2020 W3C Process Document.

1. Introduction

This section is non-normative.

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:

Example 1

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 have a stable monotonic clock, and as a result, it is subject to system clock skew.
Does not provide sub-millisecond time resolution.

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 the above issues by providing monotonically increasing time values with sub-millisecond resolution.

Note

Providing sub-millisecond resolution is not a mandatory part of this specification. Implementations may choose to limit the timer resolution they expose for privacy and security reasons, and not expose sub-millisecond timers. Use-cases that rely on sub-millisecond resolution may not be satisfied when that happens.

1.1 Use-cases

This section is non-normative.

This specification defines a few different capabilities: it provides timestamps based on a stable, monotonic clock, comparable across contexts, with potential sub-millisecond resolution.

The need for a stable monotonic clock when talking about performance measurements stems from the fact that unrelated clock skew can distort measurements and render them useless. For example, when attempting to accurately measure the elapsed time of navigating to a Document, fetching of resources or execution of script, a monotonically increasing clock with sub-millisecond resolution is desired.

Comparing timestamps between contexts is essential e.g. when synchronizing work between a Worker and the main thread or when instrumenting such work in order to create a unified view of the event timeline.

Finally, the need for sub-millisecond timers revolves around the following use-cases:

Ability to schedule work in sub-millisecond intervals. That is particularly important on the main thread, where work can interfere with frame rendering which needs to happen in short and regular intervals, to avoid user-visible jank.
When calculating the frame rate of a script-based animation, developers will need sub-millisecond resolution in order to determine if an animation is drawing at 60 FPS. Without sub-millisecond resolution, a developer can only determine if an animation is drawing at 58.8 FPS (1000ms / 16) or 62.5 FPS (1000ms / 17).
When collecting in-the-wild measurements of JS code (e.g. using User-Timing), developers may be interested in gathering sub-milliseconds timing of their functions, to catch regressions early.
When attempting to cue audio to a specific point in an animation or ensure that the audio and animation are perfectly synchronized, developers will need to accurately measure the amount of time elapsed.

1.2 Examples

This section is non-normative.

A developer may wish to construct a timeline of their entire application, including events from Worker or SharedWorker, which have different time origins. To display such events on the same timeline, the application can translate the DOMHighResTimeStamps with the help of the performance.timeOrigin attribute.

Example 2

// ---- 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);
}

2. Time Origin

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

If the global object is a Window object, the time origin MUST be equal to:
- the time when the browsing context is first created if there is no previous document;
- otherwise, the time of the user confirming the navigation during the previous document's prompt to unload algorithm, if a previous document exists and if the confirmation dialog was displayed;
- otherwise, the time of starting the navigation responsible for loading the Window object's newest Document object.
If the global object is a WorkerGlobalScope object, the time origin MUST be equal to the official moment of creation of the worker.
Otherwise, the time origin is undefined.

To get time origin timestamp, given a global object global, runs the following steps:

Assert that global's time origin is not undefined.
Let t1 be the DOMHighResTimeStamp representing the high resolution time at which the shared monotonic clock is zero.
Let t2 be the DOMHighResTimeStamp representing the high resolution time value of the shared monotonic clock at global's time origin.
Let total be the sum of t1 and t2.
Return the result of calling coarsen time with global and total.

Note

The value returned by get time origin timestamp is the high resolution time value at which time origin is zero. It may differ from the value returned by Date.now() executed at "zero time", because the former is recorded with respect to a shared monotonic clock that is not subject to system and user clock adjustments, clock skew, and so on — see § 6. Monotonic Clock.

The coarsen time algorithm, given a global object global and a DOMHighResTimeStamp timestamp, runs the following steps:

Let time resolution be 100 microseconds, or a higher implementation-defined value.
If global's relevant settings object's cross-origin isolated capability is true, set time resolution to be 5 microseconds, or a higher implementation-defined value.
In an implementation-defined manner, coarsen and potentially jitter timestamp such that its resolution will not exceed time resolution.
Return timestamp.

The relative high resolution time given a DOMHighResTimeStamp time and a global object global, runs the following steps:

Let coarse time be the result of calling coarsen time with global and time.
Let diff be the difference between coarse time and the result of calling get time origin timestamp with global.
Return diff.

The current high resolution time given a global object current global must return the result of relative high resolution time given unsafe shared current time and current global.

The unsafe shared current time must return the current value of the shared monotonic clock.

4. The `Performance` interface

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

4.1 `now()` method

The now() method MUST return the current high resolution time.

4.2 `timeOrigin` attribute

The timeOrigin attribute MUST return the value returned by get time origin timestamp for the relevant global object of the context object.

4.3 `toJSON()` method

When toJSON() is called, run [WEBIDL]'s default toJSON steps.

6. Monotonic Clock

The time values returned when calling the 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 shared monotonic clock that is shared by time origins, is monotonically increasing and not subject to system clock adjustments or system clock skew, and whose reference point is the [ECMA-262] time definition - see § 7. Privacy and Security.

Note

The user agent can reset its shared 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 shared 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 each other by exchanging messages via one of the provided messaging mechanisms - e.g. postMessage, BroadcastChannel, etc.

Note

In certain scenarios (e.g. when a tab is backgrounded), the user agent may choose to throttle timers and periodic callbacks run in that context or even freeze them entirely. Any such throttling should not affect the resolution or accuracy of the time returned by the monotonic clock.

7. Privacy and Security

7.1 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, statistical fingerprinting and microarchitectural attacks are 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, identify a particular user or reveal unrelated but same-process user data - see [CACHE-ATTACKS] and [SPECTRE] for more background.

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 minimum resolution of the DOMHighResTimeStamp type should be inaccurate enough to prevent attacks.

Where necessary, the user agent should set higher resolution values to time resolution in coarsen time's processing model, to address privacy and security concerns due to architecture or software constraints, or other considerations.

In order to mitigate such attacks user agents may deploy any technique they deem necessary. Deployment of those techniques may vary based on the browser's architecture, the user's device, the content and its ability to maliciously read cross-origin data, or other practical considerations.

These techniques may include:

Resolution reduction.
Added jitter.
Abuse detection and/or API call throttling.

Mitigating such timing side-channel attacks entirely is practically impossible: 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.

Note

Clock resolution is an unsolved and evolving area of research, with no existing industry consensus or definitive set of recommendations that applies to all browsers. To track the discussion, refer to Issue 79.

7.2 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 shared monotonic clock to the application, and that must be shared across all the browser contexts. The shared monotonic clock does not need to be tied to physical time, but is recommended to be set with respect to the [ECMA-262] definition of 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 the above mechanism in place, the shared 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 shared monotonic clock, against the current time-of-day estimate of when it is zero (i.e. the 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 the 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 the 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 unavailable entropy about the user.

High Resolution Time

W3C Working Draft 16 March 2021

Abstract

Status of This Document

1. Introduction

1.1 Use-cases

1.2 Examples

2. Time Origin

3. The `DOMHighResTimeStamp` typedef

4. The `Performance` interface

4.1 `now()` method

4.2 `timeOrigin` attribute

4.3 `toJSON()` method

5. Extensions to `WindowOrWorkerGlobalScope` mixin

5.1 The `performance` attribute

6. Monotonic Clock

7. Privacy and Security

7.1 Clock resolution

7.2 Clock drift

8. Conformance

9. IDL Index

A. Acknowledgments

B. References

B.1 Normative references

B.2 Informative references

High Resolution Time

W3C Working Draft 16 March 2021

Abstract

Status of This Document

1. Introduction

1.1 Use-cases

1.2 Examples

2. Time Origin

3. The DOMHighResTimeStamp typedef

4. The Performance interface

4.1 now() method

4.2 timeOrigin attribute

4.3 toJSON() method

5. Extensions to WindowOrWorkerGlobalScope mixin

5.1 The performance attribute

6. Monotonic Clock

7. Privacy and Security

7.1 Clock resolution

7.2 Clock drift

8. Conformance

9. IDL Index

A. Acknowledgments

B. References

B.1 Normative references

B.2 Informative references

3. The `DOMHighResTimeStamp` typedef

4. The `Performance` interface

4.1 `now()` method

4.2 `timeOrigin` attribute

4.3 `toJSON()` method

5. Extensions to `WindowOrWorkerGlobalScope` mixin

5.1 The `performance` attribute