ComPlainDate

Date-time handling in JavaScript has always been hard. While we’re all waiting for Temporal to arrive, this 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.

It may well be that ComPlainDate will stay useful even after Temporal is available — only time will tell…

API documentation at deno.land

Installation

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

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

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, in themselves 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.

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 to do is provide the name of a timezone, and that name is represented as a string, for example "Europe/Stockholm". Underscore is used instead of space, as in "Africa/Dar_es_Salaam", and some timezone names have three parts like "America/Argentina/La_Rioja".

ComPlainDate has utility functions that helps us parse, validate, sanitize and format timezone strings for the benefit of our users.

No Instant?

Surprisingly, ComPlainDate does not provide any special object representing a universal instant in time. JavaScript’s Date is basically a wrapper around a UNIX timestamp (the number of milliseconds since 1970-01-01 00:00:00 UTC) and doesn’t know about timezones. This UTC-centric aspect of Date is good for timezone-agnostic operations such as comparing universal points in time and adding or subtracting time in hours, minutes, or seconds.

Use native JavaScript Date objects with relevant utility functions until you need to do an operation that Date doesn’t support!

Plain-date

ComPlainDate provides PlainDate for operations on local calendar dates, like adding or subtracting days, months or years. All the operations you do on plain-dates are timezone agnostic.

Plain-date objects 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 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.

Plain-time

ComPlainDate provides PlainTime representing a local time-of-day. Plain-time objects are mostly used for storing and displaying a fixed time-of-day and operations on them are surprisingly uncommon.

Plain-time objects have four numeric properties (hour, minute, second, and millisecond), that can be used for operations in the rare case it’s needed. The iso property is a string in the format Thh:mm:ss.sss that is mostly used for technical purposes.

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. Of course, the toLocaleString method is best for controlled formatting in user interfaces.

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.

// If you're using Deno, you can import directly from deno.land
import {
  addDays,
  createInstant,
  daysInMonth,
  differenceInMonths,
  firstWeekDay,
  isLastDayOfMonth,
  splitDateTime,
  startOfMonth,
  startOfYear,
  WeekDay,
  weekDayNumber,
} from "https://deno.land/x/complaindate/mod.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"),
); // Note: When called without a `Date`, this produces current wall-time (now)

// 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"

// 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
// ...free from any hassle involving timezones!
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

// 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

// Quickly turn a plain-date into a UTC "instant", a JS `Date` at UTC midnight
newYearsDay.toUtcInstant(); // 2023-01-01T00:00:00.000Z

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

Why another JavaScript date-time library?

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

These 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.

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 any 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. Date doesn’t have the prettiest interface, but it makes little sense to replace it here. ComPlainDate provides some useful utilities for those operations that are relevant to do directly on instants, but honestly, they are quite few.

Timezones are represented by strings

All the information we need about any specific timezone is available in the JavaScript engine and accessible via the timezone name. So there is no need for any fancy timezone object.

Composable functions

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

Also, there are no classes here, only 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!

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 >=.

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.

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.