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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 W3C Proposed Recommendation of "High Resolution Time Specification". W3C published a technical report as a Proposed Recommendation to indicate that the document is a mature technical report that has received wide review for technical soundness and implementability and to request final endorsement from the W3C Advisory Committee. Proposed Recommendation status is described in section 7.1.1 of the Process Document.
The W3C Membership and other interested parties are invited to review the document and send comments to public-web-perf@w3.org (archived) with [HighResolutionTime] at the start of the subject line through . Advisory Committee Representatives should consult their WBS questionnaires. Note that substantive technical comments were expected during the Last Call review period that ended 10 April 2012.
Please see the Working Group's implementation report. At this stage, the Working Group encourages implementations of this specification to remove their vendor prefixes.
No changes were done to this specification during the Candidate Recommendation period.
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 Proposed 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.
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.
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. [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 "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 a time value measured relative to the
navigationStart
attribute of the
PerformanceTiming
interface [NavigationTiming], the start of navigation of the document, or a time value that represents a duration
between two DOMHighResTimeStamps
.
A DOMHighResTimeStamp
SHOULD represent a number of milliseconds accurate to a thousandth of a millisecond.
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.
typedef double DOMHighResTimeStamp;
Performance
interfacepartial 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.
As the now method returns the current time, time spent while a document is hidden [Page Visibility] or not fully active [HTML5] is included for the purpose of this 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 Karen Anderson, Nat Duca, Tony Gentilcore, Arvind Jain, James Robinson, James Simonsen, and Jason Weber to acknowledge their contributions to this work.