ComPlainDate

Date-time handling in JavaScript has always been hard, mainly because
Date
objects don't handle timezones very well.

ComPlainDate is a collection of tools for **expressive and timezone-safe
manipulation of dates and times** on top of the JavaScript features already
available in today's browsers and runtime systems. The package is
dependency-free and supports tree-shaking, making production bundle size
increase as little as 1 kB for most projects.

The central idea in ComPlainDate is to provide immutable timezone-agnostic
PlainDate and
PlainTime objects with composable utility
functions for common calendar and time related operations.

Code utilizing ComPlainDate is easier to follow because whenever timezones are
actually needed they are super obvious. And for operations where timezones are
_not_ needed, there is no way to accidentally introduce one. Timezone
information is never hidden from plain sight — ComPlainDate utilities _keep
timezones on the surface_.

API documentation

This readme aims to explain how ComPlainDate works, but doesn't contain full
explanations of all available operations. The detailed documentation and
categorized lists of available functions are available at the _jsr.io_ website:

Visit the ComPlainDate API documentation
to explore which specific utilities you can use to solve your problem.

Installation

ComPlainDate is distributed as packages via both jsr and npm:

- jsr.io/@bjuppa/complaindate
- npmjs.com/package/complaindate

Table of contents

1. Introduction
2. Creating plain-date and plain-time objects
3. Quick example
4. Working with timezone strings
5. Working with JavaScript Date objects
6. Background
7. Guiding principles
8. Limitations

Introduction

ComPlainDate provides a few special objects and a bunch of utility functions to
operate on those objects.

The main concepts we need to represent are:

- _instant_: a universal point in time.
- _calendar date_: a year, month, and day-of-month.
- _time-of-day_: a wall-time of hours, minutes, and seconds.

None of the concepts above have an inherent timezone, they are all
timezone-agnostic. But to convert an _instant_ to the corresponding local
_calendar date_ and _time-of-day_ at a specific place, and vice versa, we need
to add a supporting concept:

- _timezone_: a set of rules describing how local wall-time in an area relates
to universal time.

Let's go through how ComPlainDate supports working with each of these four
concepts, starting with the simplest but perhaps most central one.

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Modern JavaScript engines know the rules for
timezones around the globe through
Intl,
and although it's not entirely straight-forward we can tap into that knowledge
using some clever tricks. The main purpose of ComPlainDate is to abstract those
tricks away.

All we need is the name of a timezone, and those names are represented by simple
strings with underscore replacing space, like "Africa/Dar_es_Salaam".

ComPlainDate has utility functions that helps parse, validate, sanitize and
format timezone strings.

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ComPlainDate does not provide any special object representing a universal
_instant_ in time because JavaScript's
Date
already works well for timezone-agnostic operations.

ComPlainDate has a few utility functions supporting such operations and you
should aim to use native JavaScript Date objects as much as you can. When you
need to do an operation that the provided instant-utilities doesn't support it's
time to reach for the other concepts, described below.

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Plain-date objects adhere to the
ComPlainDate contract
and have three numeric properties (year, month, and day) used for most
operations.

The iso property and
string coercion
produces a string in the format yyyy-mm-dd that can be used for simple display
purposes, while the toLocaleString method is good for tailored formatting of
dates in user interfaces.

Plain-dates have a map method taking a callback function that makes it easy to
build a new plain-date that represents some modification of an existing
plain-date. They also have a pipe method that applies any number of
operations, from left to right, returning a new plain-date. The provided
plain-date utility functions can be used with map and pipe, and you are
encouraged to build your own mapper functions on top of the existing ones.

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Plain-time objects adhere to the
ComPlainTime contract
and have four numeric properties (hour, minute, second, and
millisecond), that may be used for operations, but those are surprisingly
uncommon.

For display,
string coercion
will give the shortest of the formats hh:mm / hh:mm:ss / hh:mm:ss.sss
depending on the resolution of the specific plain-time, but the toLocaleString
method is best for controlled formatting in user interfaces.

Creating plain-date and plain-time objects

Pass any _calendar-date_ or _wall-time_ shaped objects to the factory
functions PlainDate and
PlainTime:

``ts
const someDate = PlainDate({
year: 2023,
month: 7,
day: 31,
});

const someTime = PlainTime({
hour: 13,
minute: 37,
second: 59,
millisecond: 999,
});
`

Object properties may be numbers or strings and only year is required, the
others default to 1 for dates and 0 for times:

`ts
const jan1 = PlainDate({ year: "2023" }); // 2023-01-01
const midnight = PlainTime({}); // 00:00
`

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Functions
parsePlainDate and
parsePlainTime
creates objects from strings:

`ts
const xMasDay = parsePlainDate("2023-12-25");
const june1 = parsePlainDate("2023-06"); // 2023-06-01

const highResTime = parsePlainTime("01:02:03.004");
const midday = parsePlainTime("12:00");
`

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If you have a JavaScript Date object, calling
splitDateTime will
extract separate plain-date and plain-time objects for a given timezone:

`ts
// Sweden is at UTC+2 in June, so this Date represents 13:37 wall-time there
const aJsDate = new Date("2023-06-06T13:37+0200");

const [june6, time1337] = splitDateTime("Europe/Stockholm")(aJsDate);
`

A Date can also be split in UTC using
splitUtcDateTime
or the system's timezone with
splitLocalDateTime:

`ts
const [june6, time1137] = splitUtcDateTime(aJsDate);
const [aSystemDate, aSystemTime] = splitLocalDateTime(aJsDate);
`

Leaving out the Date parameter for the split-functions will extract objects
representing now, the current date and current wall-time:

`ts
const [todayInSweden, timeInSweden] = splitDateTime("Europe/Stockholm")();
const [todayInUtc, timeInUtc] = splitUtcDateTime();
const [todayInSystemTz, timeInSystemTz] = splitLocalDateTime();
`

Quick example

This will show you how to split a native JavaScript Date into separate
plain-date and plain-time objects.

We'll navigate from that plain-date to another using a _pipeline_ of operations,
and then we'll describe how to use utility functions _independently_ for
navigation and getting information about a plain-date.

The final step will merge a plain-date and a plain-time into a native JavaScript
Date, completing the circle.

`ts
// Extract a plain-date and a plain-time from any JS Date
const [june6, time1337] = splitDateTime(
"Europe/Stockholm", // Note: A timezone is required for this operation
)(
// Sweden is at UTC+2 in June, so this Date represents 13:37 wall-time there
new Date("2023-06-06T13:37+0200"),
);

// The plain-date part is an object adhering to the full ComPlainDate interface
june6; // { year: 2023, month: 6, day: 6, iso: "2023-06-06", ...}
june6.toLocaleString("en"); // "6/6/23"
june6.is({ month: 6, day: 6 }); // true

// The plain-time part is an object adhering to the full ComPlainTime interface
time1337; // { hour: 13, minute: 37, second: 0, millisecond: 0, ... }
time1337.toLocaleString("en"); // "1:37 PM"

// Apply any pipeline of operations to get a new plain-date
const midsummersEve = june6.pipe(
startOfMonth, // Go back to the 1st day of June
addDays(18), // Move to the first possible midsummer's eve candidate (June 19)
firstWeekDay(WeekDay.FRIDAY), // Find the first Friday
); // 2023-06-23

// Objects can be compared
midsummersEve.is(june6); // false
midsummersEve > june6; // true

// Utility functions can be used independently with plain-dates, for example:
const newYearsDay = startOfYear(midsummersEve); // 2023-01-01
daysInMonth(newYearsDay); // 31
isLastDayOfMonth(newYearsDay); // false
weekDayNumber(midsummersEve); // 5 (equal to WeekDay.FRIDAY)
differenceInMonths(midsummersEve)(newYearsDay); // -5

// Combine any shape of local date & time into an "instant", a JS Date
createInstant(
"Europe/Vienna", // Note: A timezone is required for this operation
)({
...newYearsDay,
...{ hour: 11, minute: 15 }, // The Wiener Musikverein is at UTC+1 in January
}); // 2023-01-01T10:15:00.000Z
`

Working with timezone strings

JavaScript throws RangeError whenever it encounters an invalid timezone name.
User input timezones will clearly need validation before use, but support for a
specific timezone name may also differ between JavaScript engines. For example,
a timezone supported in your backend may not be supported in the user's current
browser.

Before using a timezone string in frontend code, pass it through the
safeTimezone utility
to get a string guaranteed to be a valid timezone in the local system. Should
the given timezone name be unsuitable, it will return the operating system's
named timezone instead, or "UTC" if no timezone can be determined. For the
user, this should make for the best possible graceful degradation when their
preferred timezone is unavailable.

When your application doesn't support timezone as a user preference, the
localTimezone
utility can be used to retrieve a relevant timezone for the current view.
Although, be careful with server side rendering here…

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Because the timezone used may be a fallback and not what the user expects, it's
important to _always_ display the actual timezone name whenever time information
is present in the user interface.

The formatTimezone
utility will make a timezone name look pretty for the user. It replaces
underscores with spaces to give a less technical impression, for example
"Africa/Dar es Salaam" instead of "Africa/Dar_es_Salaam".

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If your user interface provides a way for users to select their preferred
timezone, use
supportedCanonicalTimezones
to get a list of all the named timezones in the system. You may even create an
endpoint that returns the timezones supported by your backend and intersect that
with the browser's timezones to really make sure no unhandled timezone is
suggested.

You may populate an HTML

Don't forget to use
localTimezone to
set a sensible initial value for the input!

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User input can be run through
sanitizeTimezone
to clean up a timezone string, removing some common user typos and converting
whitespace to underscore. The result can be checked with
isTimezone.

If you'd rather throw RangeError on failure, or want to extract a timezone
name that is part of a longer string, use
parseTimezone
directly to both sanitize and validate the result.

Working with JavaScript Date objects

JavaScript Date objects, that is _instants_, can of course be created the
usual way with
different arguments to the constructor.
It's perfect when you have a date-time as an ISO string, like you usually get
from a JSON API.

`ts
const instant = new Date(...);
`

With ComPlainDate Date objects can also be created from any date-time
shaped objects in a specified timezone with
createInstant,
createLocalInstant
and
createUtcInstant:

`ts
const noon2023Feb3InSweden = createInstant("Europe/Stockholm")({
year: 2023,
month: 2,
day: 3,
hour: 12,
minute: 0,
second: 0,
millisecond: 0,
}); // 2023-02-03T11:00:00.000Z
`

These examples combine existing plain-date and plain-time objects:

`ts
const jsDateInSweden = createInstant("Europe/Stockholm")({
...jan1,
...midday, // Sweden is at UTC+1 in January
}); // 2023-01-01T11:00:00.000Z

const jsDateInSystemTz = createLocalInstant({
...jan1,
...midday,
});

const jsDateInUtc = createUtcInstant({
...jan1,
...midday,
}); // 2023-01-01T12:00:00.000Z
`

For UTC, that last example can also be written using the
toUtcInstant
method of the plain-date object, passing an optional wall-time shaped object:

`ts
jan1.toUtcInstant(...midday); // 2023-01-01T12:00:00.000Z
`

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The formatInstant
utility generates formatting functions to reuse for consistency throughout a
user interface. It is curried in three rounds with a locale, format options, and
a timezone. Each parameter has a sensible default if left out, using the
system's locale and timezone, and including a short timezone name in the format.

`ts
const formatDateTime = formatInstant()()(); // All defaults

// Building a user specific formatter
const userLocale = "en-US";
const userTimezone = "America/New_York";
const format24hDateTimeForUser = formatInstant(userLocale)({
hourCycle: "h23",
})(userTimezone);

const aJsDate = new Date("2023-06-13T12:00Z");

// For a browser in Sweden:
formatDateTime(aJsDate); // "2023-06-13 14:00:00 CEST"

format24hDateTimeForUser(aJsDate); // "6/13/2023, 08:00:00 EDT"
`

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Use functions addTime and
subtractTime to get
a new Date object shifted some duration from an existing one. Units up to
hours make sense here because an hour is exactly 60 minutes no matter what
timezone you're in. These methods just sum up the total milliseconds before
adjusting the given Date object.

`ts
const jan1st1970 = new Date(0); // 1970-01-01T00:00:00.000Z

const laterJsDate = addTime({
hours: 25,
minutes: 61,
seconds: 61,
milliseconds: 1001,
})(jan1st1970); // 1970-01-02T02:02:02.001Z

const earlierJsDate = subtractTime({
hours: 1,
minutes: 1,
})(jan1st1970); // 1969-12-31T22:59:00.000Z
`

Adding days or larger duration units to a Date object must take timezones
into account and therefore you should first
split that Date into plain-date and plain-time objects.

Background

Most other date-time libraries either don't provide any clear strategy for
timezone handling, for example date-fns, or keep the
timezone information hidden inside date-time objects, like
Luxon does. ComPlainDate takes a lot of
inspiration from both of them (while staying clear of their pitfalls) and also
adds in some very useful ideas from the
suggested Temporal API. The
combination makes calendar operations in any timezone very easy to implement and
maintain, for frontend and backend alike.

The entire ComPlainDate API is explicitly designed to prevent developers from
making hard-to-spot mistakes and aims to remove the need for testing of timezone
related edge cases in local code. This is achieved with a few core principles:

- Explicit named timezones must be given to any
operation that actually require a timezone for results to be correct and
predictable.
- Separate plain-date and plain-time objects
and independent operations that naturally apply to each type of object.

The dangers of accidentally working in an ambiguous or incorrect timezone is
meant to be completely eliminated by design. There just is no way to
accidentally add hours to a plain-date, subtract days from a plain-time or move
to the start-of-day for a global instant in time. Every function accepts just
the shape of objects that makes sense for its purpose and all returned objects
make their meaning clear.

Guiding principles

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ComPlainDate utilities are designed to always require a named timezone for every
operation that would be ambiguous without one. The timezone is the very first
argument given to such functions, showing how important it is.

This avoids confusion caused by working with JavaScript Date or other
_DateTime_-like objects where the timezone information is hidden away. Timezones
set inside objects are especially problematic when passing those objects over
context boundaries. With ComPlainDate, developers are compelled to pass the
timezone separate from the date-time objects, making that timezone very visible.

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By keeping the calendar date and the time-of-day information in separate objects
we are free to do relevant operations on them both in an expressive way, with no
need to worry about such things as crossings into daylight savings time (DST) or
what start-of-hour means in a timezone with a half-hour offset.

The only operations where we need an explicit timezone are when we split a
universal representation of an instant (e.g. Date object) into separate
plain-date and plain-time objects, and when we merge them back together.

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The native JavaScript Date object is actually good enough for keeping
universal representations of specific instants in time, even though it doesn't
have the prettiest interface.

$3

Constructed objects include implementations of the common valueOf, toString,
toJSON, and toLocaleString methods, making their behavior and use similar to
JavaScript's Date objects when converting to primitive types. This enables
comparing objects of the same type with operators <, >, <=, and >=.

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Plural property names is an indication that an object represents a duration of
time, while singular property names are used for calendar date and time-of-day
objects.

Duration properties:

- hours
- minutes
- seconds
- milliseconds

Calendar date properties:

- year
- month
- day

Time-of-day properties:

- hour
- minute
- second
- millisecond

$3

Many operations that take some date or time representation as input don't
actually require the full object to produce correct results. For example, to
determine if a given calendar date is in a leap year, we don't care about the
month or day-of-month, only the year of the date has any significance.

Functions in ComPlainDate use object destructuring of parameters to extract only
the properties that are actually needed.

Whenever possible, relaxed objects having interchangeable number or string
type properties can also be passed to functions, making it easier to combine
user input into valid parameters.

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We've chosen not to extract type aliases for reuse within ComPlainDate, but
instead type every function parameter explicitly making IDE tools and the API
documentation show the expected object shapes up front. There exists some type
aliases but those are meant to support developers when implementing their own
utilities outside of ComPlainDate.

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Inspired by concepts from functional programming, functions are pure and
composable and operations requiring multiple arguments are implemented as
higher-order functions for currying.

Also, there are no classes here, only immutable objects adhering to interfaces
and accompanying factory functions to create them.

Please don't let this scare you, the ComPlainDate utilities are just as easy to
use in a non-functional paradigm too!

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First of all, there are no external dependencies, and there will never be any.

The base PlainDate
and PlainTime
objects are carefully composed to be _minimal loveable objects_, containing only
what is needed for a neat developer experience. The utility functions are meant
to be imported and applied with these base objects when required.

When bundle size is not an issue (i.e. server-side), you can work with full
ExPlainDate
objects if you want to call available operations directly on the plain-date
object. This may sound convenient, but it is very hard to tree-shake, making
your bundle size unnecessary big when used.

There is no extended interface for the plain-time objects, because there are
actually very few complex operations to do on a wall-time object. Plain-time
objects are most often used for display purposes, they seldom need to be
manipulated.

The footprint of a tree-shaken and compressed production build starts below
1 kB when using just the PlainDate object API. This will increase a little
with every imported utility, but you'll probably find that most projects require
very few of them.

$3

The unfortunate choice of name for Date in JavaScript makes any function with
the word date difficult to quickly assess from the name alone. Does it operate
on Date or not? Had Date been called Instant or even DateTime, the
single word date could have been used in the names of our utility functions
that operates on plain-date objects.

Now that's not the case so a deliberate decision has been made to use the longer
but less ambiguous PlainDate and PlainTime in function names. For example,
our utilities are called parsePlainDate and formatPlainDate, even though
parseDate and formatDate would have been more succinct.

Functions related to JavaScript Date objects have the word Instant in their
names whenever needed for clarity.

Limitations

Current JavaScript Date` objects support the Gregorian calendar only, and
therefore these tools have the same limitations.

The IANA timezone database is constantly
being updated and it takes a little while before changes are available in new
releases of browsers and runtime systems. This means that timezone operations
are dependent of the version of the JavaScript engine running the code. The
results of the same operation may differ between systems depending on their
version and there is no guarantee that the same code running in a browser and on
a server produces identical results.