JavaScript

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.

var date1 = new Date('December 17, 1995 03:24:00');
// Sun Dec 17 1995 03:24:00 GMT...

var date2 = new Date('1995-12-17T03:24:00');
// Sun Dec 17 1995 03:24:00 GMT...

console.log(date1 === date2);
// expected output: false;

console.log(date1 - date2);
// expected output: 0

Syntax

new Date();
new Date(value);
new Date(dateString);
new Date(year, monthIndex [, day [, hours [, minutes [, seconds [, milliseconds]]]]]);

Note: JavaScript Date objects can only be instantiated by calling JavaScript Date as a constructor: calling it as a regular function (i.e. without the newoperator) will return a string rather than a Date object; unlike other JavaScript object types, JavaScript Date objects have no literal syntax.

Parameters

Note: The argument monthIndex is 0-based. This means that January = 0 and December = 11.

Note: Where Date is called as a constructor with more than one argument, if values are greater than their logical range (e.g. 13 is provided as the month value or 70 for the minute value), the adjacent value will be adjusted. E.g. new Date(2013, 13, 1) is equivalent to new Date(2014, 1, 1), both create a date for 2014-02-01 (note that the month is 0-based). Similarly for other values: new Date(2013, 2, 1, 0, 70) is equivalent to new Date(2013, 2, 1, 1, 10) which both create a date for 2013-03-01T01:10:00.

Note: Where Date is called as a constructor with more than one argument, the specified arguments represent local time. If UTC is desired, use new Date(Date.UTC(...)) with the same arguments.

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

dateStringString value representing a date. The string should be in a format recognised 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.

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

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

dayOptional. Integer value representing the day of the month.

hoursOptional. Integer value representing the hour of the day.

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

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

millisecondsOptional. 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 for timezone offset.

• If at least two arguments are supplied, missing arguments are either set to 1 (if the day is missing) or 0 for all others.

• The JavaScript date is based on a time value that is milliseconds since midnight January 1, 1970, UTC. A day holds 86,400,000 milliseconds. The JavaScript Dateobject range is -100,000,000 days to 100,000,000 days relative to January 1, 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.prototypeAllows the addition of properties to a JavaScript Date object.Date.lengthThe value of

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

Examples

Several ways to create a Date object

The following examples show several ways to create JavaScript dates:

Note: parsing of date strings with the Date constructor (and Date.parse, they are equivalent) is strongly discouraged due to browser differences and inconsistencies.

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 milliseconds.

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;
}

var yourFunctionReturn = printElapsedTime(yourFunction);

Note: In browsers that support the Web Performance API's high-resolution time feature, Performance.now() can provide more reliable and precise measurements of elapsed time than Date.now().

Get the number of seconds since Unix Epoch

var seconds = Math.floor(Date.now() / 1000);

In this case it's important to return only a whole number (so a simple division won't do), and also to only return actually elapsed seconds (that's why this code uses Math.floor() and not Math.round()).

References

Date - JavaScript | MDN

Contributors to this page

Uros Durdevic