A date parsing and formatting library. Complete ISO 8061 date/time parsing support including quarters and years. Full PHP style date formatting.

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Dateutil is a modest collection of utility methods for manipulating dates. It works both in Node.JS and in browsers. It's goal is to provide extendable mechanisms for parsing and formatting dates, as well as other convenient date methods.
Currently, the library supports formatting of nearly full spec of PHP style dates (minus the really useless bits), and parses about the full range of ISO 8901 formats. It supports translations (for formatter) but no timezones.

Dateutil methods year, month, day, hour, min, sec, ms )

Function returns a new Date instance set to the expected date. If no arguments are given, then it returns the current date. When any arguments are given they are handled in much the same way the native date constructor does, except the first argument is always treated as a year.
It is safe to pass strings as arguments.
Please note: Months are zero based in set just as they are in native Date construction.

dateutil.isLeapYear( date / year )

Function accepts a Date, or a year, and will return true if the year is a leap year or false if not.
dateutil.isLeapYear( 1468 ) == true

dateutil.daysInMonth( date )

Function accepts a Date and returns the number of days in it's month.
dateutil.daysInMonth( new Date(1920, 3, 1) ) == 30

dateutil.isocalendar( date )

Function accepts a Date and returns a three value array containing the ISO 8901 year, week, and day (of the week), respectively.
dateutil.isocalendar( new Date(2010, 0, 3) ) == [2009, 53, 7]

dateutil.set( date, values )

Function accepts a Date and an object of values. The function returns the input date (same instance) having set the units specified in the values collection.
var mydate = new Date( 2000, 9, 16, 10, 45, 12 );
dateutil.set( mydate, { hour: 0, minute: 0, second: 0 });

result:  mydate == new Date( 2000, 9, 16, 0, 0, 0 );
The function will accept all or any of these keys: year, month, day, hour, minute, second, millisecond. and their plural forms (years, minutes, etc. ); and these shorthand variations: yr (year), mn (month), day (day), hr (hour), min (minute), sec (second), ms (millisecond).
Please note: Months are zero based in set just as they are in native Date construction.
This function tries to avoid rollover gotchas that can occur when using native members to set date values. A simplified example of this:
var d = new Date( 1999, 1, 20 );
d.setUTCDate( 30 );   // Tue Mar 02 1999 00:00:00 GMT+0000 (GMT)
d.setUTCMonth( 0 );   // Sat Jan 02 1999 00:00:00 GMT+0000 (GMT)
The dateutil.set function is takes steps to avoid the problem:
var d = new Date( 1999, 1, 20 );
dateutil.set(d, {
  'date': 30,
  'month': 0
});  // Sat Jan 30 1999 00:00:00 GMT+0000 (GMT)

var d = new Date( 1999, 1, 20 );
dateutil.set(d, {
  'month': 0,
  'date': 30
});  // Sat Jan 30 1999 00:00:00 GMT+0000 (GMT)
If you want to be sure you get the correct date then you need to use the

dateutil.parse( string )

Function accepts a string (representing a date) and will output a Date based on parsing the string.
dateutil.parse('2005-01-01') == new Date(2005, 0, 1)
The function will aways return a Date instance, valid or not. Internally it falls back to native parser if it fails to recognize the format.
A mostly complete list of recognized formats:
  • yyyy-mm-dd hh:mm:ss.ffff+0000   — (timezone is currently ignored)
  • yyyy-mm-dd hh:mm:ss.ffffZ
  • yyyy-mm-dd hh:mm:ss.ffff
  • yyyy-mm-dd hh:mm
  • yyyy-mm-ddThh:mm:ss.ffff+0000   — (timezone is currently ignored)
  • yyyy-mm-ddThh:mm:ss.ffffZ
  • yyyy-mm-ddThh:mm:ss.ffff
  • yyyy-mm-ddThh:mm
  • yyyy-mm-dd
  • yyyy-mmdd
  • yyyymmdd
  • yyyy-ddd   — year with day-of-the-year
  • yyyy-Www-d
  • yyyy-Wwwd
  • yyyy-Www
  • yyyyWww-d
  • yyyyWwwd
  • yyyyWww
  • yyyy-Qq   — year with quarter
  • yyyyQq
  • yyyy-mm
  • yyyy/mm
  • yyyy

... In addition to the formats JavaScript can natively parse.

Adding a custom parser

You may add your own parser by assigning them into dateutil._parsers. A parser is simply an object with two members:
  • test: a regular expression used to test if this date can be parsed by the parser. Keep these simple and avoid capture groups, or suffer the slowdown cost.
  • parse: a function that handles turning the string into a Date instance.

An contrived example:
// year + excel week
dateutil._parsers['year_and_excel_week'] = {
  test: /^\d{4}_[0-5]\d$/,
  parse: function ( str ) {
    var bits = str.split('_');
    var year = parseInt( bits[0], 10 );
    var dofy = parseInt( bits[1], 10 );
    return new Date( year, 0, ((dofy - 1) * 7) + 1);

// usage:
dateutil.parse('2002_31') == new Date(2002, 6, 30)

dateutil.format( date, formatstring, language )

Function accepts a Date, a PHP style format string1, and an optional language identifyer, and will return a formatted date string. Refer to the PHP docs1 for the full spec of available format characters.
var mydate = new Date( 2000, 9, 16, 10, 45, 12 );
dateutil.format( mydate, 'jS F Y' ) == "16th October 2000"
The B, Z, and I characters have been purposely omitted, and the following new characters added:
  • q — quarter of the year

The third optional parameter will get passed through the formatting system until it reaches
The format method is smart about being assigned to the Date.prototype object and will work as expected:
Date.prototype.format = dateutil.format;
new Date( 1975, 9, 16 ).format( 'Y-m-d' ) == "1975-10-16"

Adding a custom formatter

You may add your own formatter by assigning them into dateutil._formats. A parser is simply a function that takes a Date parameter, a language parameter, and returns a string.
An example that adds Swatch internet time formatter:

// Swatch Internet time
dateutil._formats['B'] = function (d) {
  var mo = 0; // This should really be: isDST * 60 + timeZoneOffsetInMinutes
  return Math.round( ( d.getUTCHours() * 3600 + 
        ( ( d.getUTCMinutes() - mo + 60 ) * 60 ) +
          d.getUTCSeconds() ) * 1000 / 86400 ) % 1000;

var mydate = new Date( 1961, 0, 3, 1, 51, 10, 1 );
dateutil.format( mydate, 'B' ) == '119'
If you call dateutil._, or recursively call formatting or other specific formatting functions for delegating work, it is important that you keep passing the second parameter so that a specified language is perserved.
An example that adds upper case 4 letter month name:
dateutil._formats['R'] = function (d,l) {
  var mont = this['F']( d, l );  // fetch translated month in "l" language
  return mont.substr( 0, 4 ).toUpperCase();

Function returns a new Date instance set to the current day.

Function returns a new Date instance set to the current moment. Uses native implementation if it exists.

dateutil.( string, language )

A string translation function. This function looks for the string argument as a key in dateutil.lang[ language ] and uses the the value if it exists, otherwise it falls back on the string argument.
You can feed the system translation strings and use it, or alternatively, if you already have another translation system then you can overwrite this function with your own translation function.
In order to use the built in system you would add translated strings like in this example (Icelandic language): = {
  'January': 'janúar',
  'February': 'febrúar',
  'March': 'mars',
  'April': 'apríl',
  'May': 'maí',
  'June': 'júní',
  // and so on ...
You should provide translations for the following strings:
  • January February March April May June July August September October November December
  • Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
  • Sunday Monday Tuesday Wednesday Thursday Friday Saturday
  • Sun Mon Tue Wed Thu Fri Sat