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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.
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/.
This is the First Public Working Draft and Last Call Working Draft for the High Resolution Time specification.
Please send comments to public-web-perf@w3.org (archived) with [HighResolutionTime] at the start of the subject line by 10 April 2012.
This document is produced by the Web Performance Working Group. The Web Performance Working Group is part of the Rich Web Clients Activity in the W3C Interaction Domain.
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 5 February 2004 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 section is non-normative.
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.
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.
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]
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.
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.
DOMHighResTimeStamp Type
The DOMHighResTimeStamp type is used to store an absolute or relative time measured from
the navigationStart attribute of the
PerformanceTiming
interface [NavigationTiming], the start of navigation of the document.
A DOMHighResTimeStamp represents a number of milliseconds accurate to at least a tenth of a millisecond.
typedef double DOMHighResTimeStamp;
Performance interface
partial interface Performance {
DOMHighResTimeStamp now();
};
now methodThe now method MUST
return a DOMHighResTimeStamp representing the number of milliseconds
from the navigationStart attribute of the
PerformanceTiming
interface [NavigationTiming],
the start of navigation of the document, to the occurrence of the call to the now method.
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.
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.
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.