Date

Creates a JavaScript Date instance that represents a single moment in time. Date objects are based on a time value that is the number of milliseconds since 1 January, 1970 UTC.

Syntax

new Date();
new Date(value);
new Date(dateString);
new Date(year, month[, day[, hour[, minutes[, seconds[, milliseconds]]]]]);

Parameters

value

Integer value representing the number of milliseconds since 1 January 1970 00:00:00 UTC, with leap seconds ignored (Unix Epoch; but consider that most Unix time stamp functions count in seconds).

dateString

String value representing a date. The string should be in a format recognized by the Date.parse() method (IETF-compliant RFC 2822 timestamps and also a version of ISO8601).

Note: parsing of date strings with the Date constructor (and Date.parse, they are equivalent) is strongly discouraged due to browser differences and inconsistencies. Support for RFC 2822 format strings is by convention only. Support for ISO 8601 formats differs in that date-only strings (e.g. "1970-01-01") are treated as UTC, not local.

year

Integer value representing the year. Values from 0 to 99 map to the years 1900 to 1999. See the example below.

month

Integer value representing the month, beginning with 0 for January to 11 for December.

day

Optional. Integer value representing the day of the month.

hour

Optional. Integer value representing the hour of the day.

minute

Optional. Integer value representing the minute segment of a time.

second

Optional. Integer value representing the second segment of a time.

millisecond

Optional. Integer value representing the millisecond segment of a time.

Description

• If no arguments are provided, the constructor creates a JavaScript Date object for the current date and time according to system settings.
• If at least two arguments are supplied, missing arguments are either set to 1 (if day is missing) or 0 for all others.
• The JavaScript date is based on a time value that is milliseconds since midnight 01 January, 1970 UTC. A day holds 86,400,000 milliseconds. The JavaScript Date object range is -100,000,000 days to 100,000,000 days relative to 01 January, 1970 UTC.
• The JavaScript Date object provides uniform behavior across platforms. The time value can be passed between systems to create a date that represents the same moment in time.
• The JavaScript Date object supports a number of UTC (universal) methods, as well as local time methods. UTC, also known as Greenwich Mean Time (GMT), refers to the time as set by the World Time Standard. The local time is the time known to the computer where JavaScript is executed.
• Invoking JavaScript Date as a function (i.e., without the new operator) will return a string representing the current date and time.

Properties

Date.prototype

Allows the addition of properties to a JavaScript Date object.

Date.length

The value of Date.length is 7. This is the number of arguments handled by the constructor.

Methods

Date.now()

Returns the numeric value corresponding to the current time - the number of milliseconds elapsed since 1 January 1970 00:00:00 UTC, with leap seconds ignored.

Date.parse()

Parses a string representation of a date and returns the number of milliseconds since 1 January, 1970, 00:00:00, UTC, with leap seconds ignored.

Note: Parsing of strings with Date.parse is strongly discouraged due to browser differences and inconsistencies.

Date.UTC()

Accepts the same parameters as the longest form of the constructor (i.e. 2 to 7) and returns the number of milliseconds since 1 January, 1970, 00:00:00 UTC, with leap seconds ignored.

JavaScript Date instances

All Date instances inherit from Date.prototype. The prototype object of the Date constructor can be modified to affect all Date instances.

Date.prototype Methods

Examples

Several ways to create a Date object

The following examples show several ways to create JavaScript dates:

var today = new Date();
var birthday = new Date('December 17, 1995 03:24:00');
var birthday = new Date('1995-12-17T03:24:00');
var birthday = new Date(1995, 11, 17);
var birthday = new Date(1995, 11, 17, 3, 24, 0);

Two digit years map to 1900 - 1999

In order to create and get dates between the years 0 and 99 the Date.prototype.setFullYear() and Date.prototype.getFullYear() methods should be used.

var date = new Date(98, 1); // Sun Feb 01 1998 00:00:00 GMT+0000 (GMT)

// Deprecated method, 98 maps to 1998 here as well
date.setYear(98);           // Sun Feb 01 1998 00:00:00 GMT+0000 (GMT)

date.setFullYear(98);       // Sat Feb 01 0098 00:00:00 GMT+0000 (BST)

Calculating elapsed time

The following examples show how to determine the elapsed time between two JavaScript dates in millisconds.

Due to the differing lengths of days (due to daylight saving changeover), months and years, expressing elapsed time in units greater than hours, minutes and seconds requires addressing a number of issues and should be thoroughly researched before being attempted.

// using Date objects
var start = Date.now();

// the event to time goes here:
doSomethingForALongTime();
var end = Date.now();
var elapsed = end - start; // elapsed time in milliseconds

// using built-in methods
var start = new Date();

// the event to time goes here:
doSomethingForALongTime();
var end = new Date();
var elapsed = end.getTime() - start.getTime(); // elapsed time in milliseconds

// to test a function and get back its return
function printElapsedTime(fTest) {
  var nStartTime = Date.now(),
      vReturn = fTest(),
      nEndTime = Date.now();

  console.log('Elapsed time: ' + String(nEndTime - nStartTime) + ' milliseconds');
  return vReturn;
}

yourFunctionReturn = printElapsedTime(yourFunction);

Specifications

Browser compatibility

[1] Some browsers can have issues when parsing dates: 3/14/2012 blog from danvk Comparing FF/IE/Chrome on Parsing Date Strings

[2] ISO8601 Date Format is not supported in Internet Explorer 8, and other version can have issues when parsing dates