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 deno.land website:

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

Installation

ComPlainDate is distributed as an npm package as well as a Deno module:

• npmjs.com/package/complaindate
• deno.land/x/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.

Timezones are just strings

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.

What, no Instant?

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.

PlainDate for calendar dates

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.

PlainTime for time-of-day

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:

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:

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

Extraction from strings

Functions parsePlainDate and parsePlainTime creates objects from strings:

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");

Extraction from JavaScript Date objects

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

// 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:

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:

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.

// 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…

Show the timezone name in the user interface

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".

Guided timezone preference input

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 <datalist id="availableTimezones"> with all relevant timezones, enabling an ordinary <input type="text" list="availableTimezones"> to become an autocomplete "combobox" for the user to select from. See the datalist documentation on MDN for details and examples.

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

Validating timezones

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.

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:

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:

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:

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

Displaying a Date to users

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.

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"

Operations on Date

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.

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

Explicit named timezones

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.

Separate plain-date and plain-time objects

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.

Instants are represented by Date objects

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.

Coerce objects to relevant primitive types

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 >=.

Object shape carries meaning

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

Allow partial and relaxed objects where possible

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.

Avoid type aliases

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.

Composable functions

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!

Allow for the smallest possible bundle size

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.

Unambiguous function names

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.