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.
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 http://www.w3.org/TR/.
W3C publishes a
Candidate
Recommendation to indicate that the document is believed to be
stable and to encourage implementation by the developer community.
The entrance criteria for this document to enter the Proposed
Recommendation stage is to have a minimum of two independent and
interoperable user agents that implementation all the features of
this specification, which will be determined by passing the user
agent tests defined in the test suite developed by the Working Group.
The Working Group does not expect to advance to Proposed
Recommendation prior to . A preliminary
implementation report is available and will be updated during the
Candidate Recommendation period. This is a work in progress and may
change without any notices.
The Working Group intends to gain implementation experience before
recommending implementations to remove their vendor prefixes.
Publication as a Candidate Recommendation 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 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.
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.
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,
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.
In order 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.
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.
2 Conformance
requirements
All diagrams, examples, and notes in this specification are non-normative,
as are all sections explicitly marked non-normative. Everything else in this
specification is normative.
The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this document
are to be interpreted as described in RFC 2119.
For readability, these words do not appear in all uppercase letters in this specification.
Requirements phrased in the imperative as part of algorithms (such as
"strip any leading space characters" or "return false and abort these steps")
are to be interpreted with the meaning of the key word ("must", "should",
"may", etc) used in introducing the algorithm.
Some conformance requirements are phrased as requirements on attributes,
methods or objects. Such requirements are to be interpreted as requirements
on user agents.
Conformance requirements phrased as algorithms or specific steps may be
implemented in any manner, so long as the end result is equivalent. (In
particular, the algorithms defined in this specification are intended to be
easy to follow, and not intended to be performant.)
The IDL fragments in this specification must be interpreted as
required for conforming IDL fragments, as described in the Web IDL
specification. [Web IDL]
3 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 DOM is used to refer to the API set made available to scripts in
Web applications, and does not necessarily imply the existence of an actual
Document object or of any other Node objects as
defined in the DOM Core specifications.
A DOM attribute is said to be getting when its value is being
retrieved (such as by author script), and is said to be setting when
a new value is assigned to it.
The term "JavaScript" is used to refer to
ECMA-262,
rather than the official term ECMAScript, since the term JavaScript is more widely known.
4 High Resolution Time
4.1 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.
A DOMHighResTimeStamp SHOULD represent a number of milliseconds accurate to a thousandth of a millisecond.
Note
If the User Agent is unable to provide a time value accurate to a thousandth of a millisecond due to hardware or software constraints, the User Agent
can represent a DOMHighResTimeStamp as a number of milliseconds accurate to a millisecond.
As the now method returns the current time, time spent while a document is hidden or
not fully active is
included for the purpose of this method.
4.4 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.
4.5 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.
Document Object Model Level 3 Core
Specification, A. Le Hors, et al., Editors. World
Wide Web Consortium, 7 April 2004. This version of the Document
Object Model Level 3 Core Recommendation is
http://www.w3.org/TR/2004/REC-DOM-Level-3-Core-20040407. The latest version of
DOM Core is available at
http://www.w3.org/TR/domcore/.
Navigation Timing, Zhiheng Wang, Editor. World Wide Web Consortium, March 2012. This version of the Navigation Timing specification is available from http://www.w3.org/TR/2012/CR-navigation-timing-20120313/. The latest version of Navigation Timing is available at http://www.w3.org/TR/navigation-timing/.
Web IDL, Cameron McCormack, Editor. World Wide Web Consortium, April 2012. This version of the Web IDL specification is available from http://www.w3.org/TR/2012/CR-WebIDL-20120419/. The latest version of Web IDL is available at http://www.w3.org/TR/WebIDL/.
Acknowledgements
I would like to sincerely thank James Robinson, Tony Gentilcore, Nat Duca, James Simonsen, Karen Anderson, Arvind Jain, and Zhiheng Wang
to acknowledge their contributions to this work.