Date Extra library add/subtract/diff/format etc dates.

elm-package install rluiten/elm-date-extra 9.3.1


Date Extra Package


I have discovered that I managed to copy concepts from NodaTimes Period concept to Duration and there Duration concept to Period I got it backwards. Which I am sure is not helping anyone with back ground in nodatime doing the right thing with elm-date-extra.

  • I am not sure if I should swap them in elm-date-extra so they match the NodaTime terms at the moment.


An Elm language package for working with dates and times.

See bottom of this document for important history notes.

Code format

This project uses elm-format for formatting.


  • flexible formatting of dates into strings
  • It has support for outputting dates with other offsets than local by deriving a timezone offset for a dates.
  • simple i18n support for long and short Day and Month names.
  • see
  • compare dates
  • add or subtract time periods on a date
  • When modifying dates using Duration Day, Week, Month, Year the library compensates for daylight saving hour varations. It should behave very much like momentjs in its add subtract date field functions.
  • Date diff
  • For Period to Period.DeltaRecord date fields
  • millisecond, second, minute, hour, day, week
  • For Duration to Duration.DeltaRecord date fields
    • millisecond, second, minute, hour, day, month, year
  • Easy ways to move a date to start 'startOfTime' or end 'endOfTime' to adjusted a date down or up to then end of a given date unit boundary
  • Set date field module.

Example of formatting Dates

import Date.Extra.Config.Config_en_au exposing (config)
import Date.Extra.Format as Format exposing (format, formatUtc, isoMsecOffsetFormat)

displayString1 =
    Result.withDefault "Failed to get a date." <|
            (format config config.format.dateTime)
            (Date.fromString "2015-06-01 12:45:14.211Z")

displayString2 =
    Result.withDefault "Failed to get a date." <|
            (formatUtc config isoMsecOffsetFormat)
            (Date.fromString "2015-06-01 12:45:14.211Z")

It is a start but it is by no means complete and there maybe many good things we can do to make it harder to do wrong things by leveraging Elm's types.

There is some fudging done to get timezone offset available in Elm without needing it be added at a native level. It may be a good idea in the future to introduce more access to javascript for more date functions or information.

Where did this come from.

This was created based upon code extracted from one of Robin's projects and also from Luke's and put into

The date time format code originally came from however I have modified it and hence any problems you discover with it in this package should be initially raised with me.

While there are tests they can't possible cover the range of what can be done with dates. In addition Elm is at the mercy of the underlying javascript Date implementation and that is likely to have impact as well.

There be Dragons here. "Date's in General"

Please be warned that there are many ways to manipulate dates that produce basically incorrect results. This library does not yet have much in the way of prevention of doing the wrong thing.

Dates, times, timezones and offsets can make working with dates a challenge.

This library is quite new and even though it has tests and written in Elm it might eat your lunch if you are not careful.


It is hoped that with feedback from users and reviewers with deep Type-zen it will be possible to improve the API to reduce the chances of doing the wrong thing with out realising it.

Feedback that is of interest.

  • Additional locale for the Configs section.
  • Suggestions to improve API and or package structure or Types.
  • Particularly interested in improvements that might make it safer and easier to work with dates.
  • More Examples for example folder
  • Improved documentation.
  • Bugs.
  • Please try to include sufficient detail to reproduce.
  • Better yet create a test and submit a pull request, even if you cant figure out how to fix it.
  • More tests.
  • That demonstrate issues.
  • That fill a short fall in existing tests..


I think there may be value in creating Types for each type of date. Types as covered in the Noda Time documentation such as Instant, LocalTime , LocalDate, LocalDateTime, DateTimeZone, ZonedDateTime, Period and Duration.

This library has a simple Period and Duration modules at the moment, I hope this is a step in the right direction and does not muddy the water.

In the long run this may require writing a date parser and introducing Elm native time zone structures in.

People Using this library.

  • Currently Robin on a new far from finished project. Only put this here because this section would be empty with out it.
  • Feel free to contact me to let me know you are using this library.

Things to think about for future development, not really a road map.

  • Consider a range checking year inputs, javascript getFullYear() only claims to work for years 1000 to 9999, this is probably a reasonable range for range checking. In this case also check ranges for fromTime and toTime functions ?

Useful references

Many ideas and concepts shamelessly borrowed from the following.


This uses elm-test for testing so install it if you dont have it.

  • npm install -g elm-test

To run Tests

  • elm-test


Only major (and recent) changes are listed here.

  • 2017/01/31 v8.2.0


  • Changes the value of Format.isoMsecOffsetFormat
  • It now usses the %:z format token for zone offset which means they now include ":" this now appears to be a better choice as it is more generally parsed by browsers see issue.

  • Changes result of Format.isoString
  • which uses Format.isoMsecOffsetFormat.

  • 2016/07/25 8.0.0

  • Add new format codes to format output day of month with a language idiom suffix.

    • In English eg for 2015/04/02 '%-@d' outputs '2nd', '%@e' outputs ' 2nd'
  • Only English has an implementation in place, I have no idea if this idiom exists in other languages so French, Finish and Polish currently only output day of month as decimal integer.

Copyright (c) 2016-2018 Robin Luiten