@meltwater/tau 2.0.0


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Functional time and date tools for JavaScript inspired by Luxon. Plays well with φ.


Exports the Luxon API in a functional programming style along with the custom functions defined in this package so they all may be imported from the same place.


Add this as a dependency to your project using npm with

$ npm install @meltwater/tau

or using Yarn with

$ yarn add @meltwater/tau


See the complete API documentation and working examples.

import { compose } from '@meltwater/phi' // standard compose function
import { toIso, fromNow } from '@meltwater/tau'

const sinceStartOfToday = compose(

sinceStartOfToday() //=> '12 hours ago'
import { compose, gt } from '@meltwater/phi'
import { fromIso, diffNow, days } from '@meltwater/tau'

const isIsoMoreThanNDaysOld = n => compose(

const isIsoMoreThan3DaysOld = isIsoMoreThanNDaysOld(3)

isIsoMoreThan3DaysOld('2018-01-24T20:27:49.288Z') //=> true

Luxon Functional API

In general, each public static method, member, and method in the Luxon API is exported at the top level with the same name. All functions are curried and the argument order is reversed. Exceptions are documented below.

Naming conventions

  • Acronyms follow normal camelcase rules are and not uppercase, e.g., DateTime.isInDST is exported as isInDst.
  • For instance methods, the last argument corresponds to the instance (this), e.g., DateTime.fromMillis(0).toIso() becomes toIso(fromMillis(0)).
  • Some names are changed to avoid conflicts.
  • Some signatures are changed for compatibility with the functional style e.g., all variadic methods.
  • Any function which takes options has both a *withOptions variant which takes options as its first argument, and a normal variant with one less argument bound to the default options.
  • Some Luxon methods have arguments with default values. In these cases, extra functions are provided corresponding to those defaults.
  • Functions with a Utc suffix have been wrapped with setZone('utc').


  • diff, diffWithOptions, diffMillis: the last argument is an array [x, y] and computes the difference x - y.
  • diffNow, diffNowWithOptions and diffNowMillis compute the difference x - now (where x is the last argument).
Renamed or added
  • DateTime.fromFormat -> fromFormatWithOptions
    • fromFormat
    • fromFormatUtc
  • DateTime.fromFormatExplain -> fromFormatExplainWithOptions
    • fromFormatExplain
    • fromFormatExplainUtc
  • DateTime.fromHTTP -> fromHttpWithOptions
    • fromHttp
    • fromHttpUtc
  • DateTime.fromISO -> fromIsoWithOptions
    • fromIso
    • fromIsoUtc
  • DateTime.fromJSDate -> fromJsDateWithOptions
    • fromJsDate
    • fromJsDateUtc
  • DateTime.fromMillis -> fromMillisWithOptions
    • fromMillis
    • fromMillisUtc
  • DateTime.fromSeconds -> fromSecondsWithOptions
    • fromSeconds
    • fromSecondsUtc
  • DateTime.fromRFC2822 -> fromRfc2822WithOptions
    • fromRfc2822
    • fromRfc2822Utc
  • DateTime.fromSQL -> fromSqlWithOptions
    • fromSql
    • fromSqlUtc
  • DateTime.diff -> diffWithOptions
    • diff
    • diffMillis
  • DateTime.diffNow -> diffNowWithOptions
    • diffNow
    • diffNowMillis
  • DateTime.resolvedLocaleOpts -> resolvedLocaleOptsWithOptions
    • resolvedLocaleOpts
  • DateTime.setZone -> setZoneWithOptions
    • setZone
    • setZoneLocal
    • setZoneUtc
  • DateTime.toISOTime -> toIsoTimeWithOptions
    • toIsoTime
  • DateTime.toLocaleParts -> toLocalePartsWithOptions
    • toLocaleParts
  • DateTime.toLocaleString -> toLocaleStringWithOptions
    • toLocaleString
  • DateTime.toRelative -> toRelativeWithOptions
    • toRelative
  • DateTime.toRelativeCalendar -> toRelativeCalendarWithOptions
    • toRelativeCalendar
  • DateTime.toSQL -> toSqlWithOptions
    • toSql
  • DateTime.toSQLTime -> toSqlTimeWithOptions
    • toSqlTime
  • DateTime.toUTC -> toUtcWithOptions
    • toUtc
    • toUtcZero
  • DateTime.toJSON -> toJson
  • DateTime.toBSON -> toBson
  • Zone.offset -> offsetAt
  • Zone.offsetName -> offsetNameAtWithOptions
    • offsetName
Signature changed
  • DateTime.local(...) -> local([...]) (order preserved)
  • DateTime.utc(...) -> utc([...]) (order preserved)
  • DateTime.min(x, y, ...) -> min(x, y) (exactly two arguments)
  • DateTime.min(x, y, ...) -> max(x, y) (exactly two arguments)


  • Duration.fromISO -> durationFromIsoWithOptions
    • durationFromIso
  • Duration.fromMillis -> durationFromMillisWithOptions
    • durationFromMillis
  • Duration.fromObject -> durationFromObject
  • Duration.invalid -> durationInvalid
Signature changed
  • duration.shiftTo(...) -> shiftTo([...], duration) (order preserved)


Renamed or Added
  • Interval.fromISO -> intervalFromIsoWithOptions
    • intervalFromIso
  • Interval.invalid -> intervalInvalid
  • Interval.toDuration -> toDurationWithOptions
    • toDuration
    • toDurationMillis
  • Interval.hasSame -> hasSameEndpoints
Signature changes
  • Interval.fromDateTimes(start, end) -> fromDateTimes([start, end])
  • Interval.splitAt(...) -> splitAt([...], interval) (order preserved)
  • Interval.difference(...) -> difference([...], interval) (order preserved)


  • Info.eras -> erasWithOptions
    • eras
    • erasShort
    • erasLong
  • Info.hasDST -> hasDst
    • hasDstLocal
  • Info.isValidIANAZone -> isValidIanaZone
    • isValidIanaZoneLocal
  • Info.meridiems -> meridiemsWithOptions
    • meridiems
  • Info.months -> calendarMonthsWithOptions
    • calendarMonths
    • calendarMonthsLong
  • Info.monthsFormat -> monthsFormatWithOptions
    • monthsFormat
    • monthsFormatLong
  • Info.weekdays -> weekdaysWithOptions
    • weekdays
    • weekdaysLong
  • Info.weekdaysFormat -> weekdaysFormatWithOptions
    • weekdaysFormat
    • weekdaysFormatLong


  • Getters are exported as functions with the corresponding Luxon name, e.g., defaultLocale().
  • Setters are prefixed with set, e.g., setDefaultLocale('utc').

Development Quickstart

$ git clone https://github.com/meltwater/tau.git
$ cd tau
$ nvm install
$ yarn

Run each command below in a separate terminal window:

$ yarn run watch
$ yarn run test:watch

Development and Testing

Source code

The tau source is hosted on GitHub. Clone the project with

$ git clone git@github.com:meltwater/tau.git


You will need Node.js with npm, Yarn, and a Node.js debugging client.

Be sure that all commands run under the correct Node version, e.g., if using nvm, install the correct version with

$ nvm install

Set the active version for each shell session with

$ nvm use

Install the development dependencies with

$ yarn


CircleCI should already be configured: this section is for reference only.

The following environment variables must be set on CircleCI:

  • NPM_TOKEN: npm token for installing and publishing packages.
  • NPM_TEAM: npm team to grant read-only package access (format org:team, optional).
  • CODECOV_TOKEN: Codecov token for uploading coverage reports (optional).

These may be set manually or by running the script ./.circleci/envvars.sh.


For documentation publishing, CircleCI must have an SSH key for github.com added as a deploy key with write access on the GitHub repository.

Development tasks

Primary development tasks are defined under scripts in package.json and available via yarn run. View them with

$ yarn run

Production build

Lint, test, and transpile the production build to dist with

$ yarn run dist
Publishing a new release

Release a new version using npm version. This will run all tests, update the version number, create and push a tagged commit, and trigger CircleCI to publish the new version to npm.

  • Update the CHANGELOG before each new release after version 1.
  • New versions are released when the commit message is a valid version number.
  • Versions are only published on release branches: master branch or any branch matching ver/*.
  • If branch protection options are enabled, you must first run npm version on a separate branch, wait for the commit to pass any required checks, then merge and push the changes to a release branch.
  • Do not use the GitHub pull request button to merge version commits as the commit tagged with the new version number will not match after merging.


See the full documentation on using examples.

View all examples with

$ yarn run example


Linting against the JavaScript Standard Style and JSON Lint is handled by gulp.

View available commands with

$ yarn run gulp --tasks

Run all linters with

$ yarn run lint

In a separate window, use gulp to watch for changes and lint JavaScript and JSON files with

$ yarn run watch

Automatically fix most JavaScript formatting errors with

$ yarn run format


Unit and integration testing is handled by AVA and coverage is reported by Istanbul and uploaded to Codecov.

  • Test files end in .spec.js.
  • Unit tests are placed under lib alongside the tested module.
  • Integration tests are placed in test.
  • Static files used in tests are placed in fixtures.

Watch and run tests on changes with

$ yarn run test:watch

If using AVA snapshot testing, update snapshots with

$ yarn run test:update

Generate a coverage report with

$ yarn run report

An HTML version will be saved in coverage.

Debugging tests

Create a breakpoint by adding the statement debugger to the test and start a debug session with, e.g.,

$ yarn run test:inspect lib/unix.spec.js

Watch and restart the debugging session on changes with

$ yarn run test:inspect:watch lib/unix.spec.js


The author and active contributors may be found in package.json,

$ jq .author < package.json
$ jq .contributors < package.json

To submit a patch:

  1. Request repository access by submitting a new issue.
  2. Create your feature branch (git checkout -b my-new-feature).
  3. Make changes and write tests.
  4. Commit your changes (git commit -am 'Add some feature').
  5. Push to the branch (git push origin my-new-feature).
  6. Create a new Pull Request.


This npm package is licensed under the MIT license.


This software is provided by the copyright holders and contributors "as is" and any express or implied warranties, including, but not limited to, the implied warranties of merchantability and fitness for a particular purpose are disclaimed. In no event shall the copyright holder or contributors be liable for any direct, indirect, incidental, special, exemplary, or consequential damages (including, but not limited to, procurement of substitute goods or services; loss of use, data, or profits; or business interruption) however caused and on any theory of liability, whether in contract, strict liability, or tort (including negligence or otherwise) arising in any way out of the use of this software, even if advised of the possibility of such damage.