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

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High Resolution Time Level 2 builds on the first version of High Resolution Time and includes:

This document was published by the Web Performance Working Group as a Working Draft. This document is intended to become a W3C Recommendation. If you wish to make comments regarding this document, please send them to public-web-perf@w3.org (subscribe, archives) with [hr-time] at the start of your email's subject. All comments are welcome.

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Table of Contents

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 log a positive number, negative number, or zero.

Example 1
var mark_start = Date.now();
doTask(); // Some task
if (window.console) window.console.log('Duration of task: ' + (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() as it is genuinely useful in determining the current value of the calendar time and has a long history of usage. The DOMHighResTimeStamp type and the now method of the Performance interface resolve the issues summarized in this section by providing a monotonically increasing time value in 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 a different time origin. To display such events on the same timeline, the application can adjust the DOMHighResTimeStamps from the worker with the workerStart attribute.

Example 2
<!-- Timing task in document -->
var mark_start = performance.now();
doTaskInDocument(); // Some other task
var mark_end = performance.now();
var document_entry = {'task': 'Some document task',
                      'start_time': mark_start,
                      'end_time': mark_end};

<!-- Timing task in dedicated or shared worker -->
var worker = new SharedWorker('js');
worker.port.onmessage = function (event) {
  var worker_entry = event.data;

  // adjust timestamps to account for different time-origin
  worker_entry.start_time -= worker.workerStart;
  worker_entry.end_time -= worker.workerStart;
  plotTimeline([worker_entry, document_entry]);

Note that the WorkerGlobalScope doesn't have access to its workers' workerStart values. If needed, the document can send a message to the worker containing its workerStart value.

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]

2.1 Terminology

The construction "a Foo object", where Foo is actually an interface, is sometimes used instead of the more accurate "an object implementing the interface Foo.

The term "JavaScript" is used to refer to ECMA262, rather than the official term ECMAScript, since the term JavaScript is more widely known. [ECMA-262]

2.2 High Resolution Time

2.2.1 Introduction

This section is non-normative.

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

2.2.2 Time Origin

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


The time value of the start of navigation of the document in an attribute of type DOMHighResTimeStamp is equal to 0. The same time value described with an attribute of type DOMTimeStamp is equal to the navigationStart attribute of the PerformanceTiming interface [navigation-timing].

2.2.3 The DOMHighResTimeStamp Type

The DOMHighResTimeStamp type is used to store a time value measured relative from the time origin or a time value that represents a duration between two DOMHighResTimeStamps.

typedef double DOMHighResTimeStamp;

A DOMHighResTimeStamp SHOULD represent a time in milliseconds accurate to a microsecond.


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

2.2.4 The performance.now() method

interface Performance : EventTarget {
    DOMHighResTimeStamp now ();
}; Methods

The now method MUST return a DOMHighResTimeStamp representing the time in milliseconds from the time origin to the occurrence of the call to the now method.

No parameters.
Return type: DOMHighResTimeStamp

2.2.5 The workerStart attribute

partial interface AbstractWorker {
    readonly    attribute DOMHighResTimeStamp workerStart;
}; Attributes
workerStart of type DOMHighResTimeStamp, readonly

The workerStart attribute MUST return a DOMHighResTimeStamp representing the difference between the time origin of WorkerGlobalScope [workers] and the time origin of the current document.


The value returned by workerStart may be negative if a SharedWorkerGlobalScope already existed for the SharedWorker prior to the document's time origin.

2.2.6 Monotonic Clock

The time values returned when calling the now method MUST be 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 now method MUST never be negative.

2.2.7 Privacy and Security

Statistical fingerprinting is a privacy concern where a malicious web site may determine whether a user has visited a third-party web site by measuring the timing of cache hits and misses of resources in the third-party web site. Though the now method of the Performance interface returns time data to a greater accuracy than before, it does not make this privacy concern significantly worse than it was already.

2.3 Acknowledgments

I would like to sincerely thank Karen Anderson, Nat Duca, Tony Gentilcore, Arvind Jain, and Jason Weber to acknowledge their contributions to this work.

A. References

A.1 Normative references

S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
Cameron McCormack. Web IDL. 19 April 2012. W3C Candidate Recommendation. URL: http://www.w3.org/TR/WebIDL/
Ian Hickson; Robin Berjon; Steve Faulkner; Travis Leithead; Erika Doyle Navara; Edward O'Connor; Silvia Pfeiffer. HTML5. 28 October 2014. W3C Recommendation. URL: http://www.w3.org/TR/html5/
Ian Hickson. Web Workers. 1 May 2012. W3C Candidate Recommendation. URL: http://www.w3.org/TR/workers/

A.2 Informative references

ECMAScript Language Specification, Edition 5.1. June 2011. URL: http://www.ecma-international.org/publications/standards/Ecma-262.htm
Zhiheng Wang. Navigation Timing. 17 December 2012. W3C Recommendation. URL: http://www.w3.org/TR/navigation-timing/