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.
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/.
High Resolution Time Level 3 replaces the second version of High Resolution Time [HR-TIME-2] and includes:
This document was published by the Web Performance Working Group as a Candidate Recommendation. This document is intended to become a W3C Recommendation. If you wish to make comments regarding this document, the GitHub repository is preferred for discussion of this specification. There is also a public mailing list
public-web-perf@w3.org
(archives). When sending e-mail, please use [hr-time-3] at the start of your email's subject. All comments are welcome.
Publication as a First Public 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.
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 taskvar 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,
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.
When calculating the animation state from script, developers will need to accurately know the amount of time that has elapsed in the animation in order to properly update the next scene of the animation.
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 or 62.5 FPS.
When attempting to cue audio to a specific point in an animation or ensure that the audio is synchronized with the animation, developers will need to accurately know the amount of time elapsed in the animation and audio.
When multiple contexts need to synchronize work with sub-millisecond resolution (e.g. when using dedicated or
shared workers to drive animation, audio, etc., in a renderer context), or to create a unified view of the event timeline.
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.
1.1 Examples
This section is non-normative.
A developer may wish to construct a timeline of their entire application, including events from dedicated or
shared workers, 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.
Example 2
// ---- worker.js -----------------------------// Shared worker script
onconnect = function(e) {
var port = e.ports[0];
port.onmessage = function(e) {
// Time execution in workervar 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 documentvar 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 originvar 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. Conformance
As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.
The key words MUST and SHOULD are to be interpreted as described in [RFC2119].
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]
3. Time Origin
The time origin is the time value from which time is measured:
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
7.Monotonic Clock.
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 time values returned when calling the Performance.now method on Performance objects with the same time originMUST 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.timeOriginMUST 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
8.Privacy and Security.
Note
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.
8. Privacy and Security
8.1 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 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.
8.2 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 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.
A. 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.
B. References
B.1 Normative references
[HTML51]
Steve Faulkner; Arron Eicholz; Travis Leithead; Alex Danilo. W3C. HTML 5.1. 15 September 2016. W3C Proposed Recommendation. URL: https://www.w3.org/TR/html51/