Calendar Date Type for JavaScript/TypeScript

What is it?

A JavaScript calendar date class, that can represent any calendar date from January 1, 1 CE to December 31, 9999. Dates are internally represented as a single Number for very high performance and memory efficiency.

This is a tiny single-file library with no dependencies and is just 1.35 kB when minified and gzipped. It's also by far the most performant implementation I know of (see benchmarks).

For dates prior to October 1582, it assumes a back-projected ("proleptic") Gregorian calendar, as if that were always the calendar in use at the time.

Why do you want this?

A calendar date like "August 5, 2023" is a very different thing from a timestamp (an exact moment in time). But many APIs conflate the two.

Python very correctly distinguishes between them with its date and datetime classes. But many JavaScript APIs (including the core JavaScript language spec) conflate these two concepts into a single Date class, which can create a lot of potential for bugs to creep into your application.

In my opinion, you should avoid using JavaScript's built-in Date() type for representing calendar dates, because it only works if you are extremely careful to always use UTC, both when constructing the Date and when displaying it.

e.g. on my system, new Date("2021-03-01").toString() gives Feb 28 2021... - the date "March 1" has been accidentally changed to "Feb 28" through code that looks perfectly reasonable. In this case, the Date is correctly constructed using UTC, but is formatted using local time. Likewise, in some timezones you can see new Date(2021, 2, 1).toISOString().substring(0,10) (March 1) will be printed as "2021-02-28" (Feb 28) through the opposite problem - the date is constructed using local time but printed using UTC.

🌟 There is going to be a new Temporal.PlainDate in future versions of JavaScript which will solve all of these problems, but it is not yet fully implemented in major runtimes like V8. For now, you need to use a pollyfill or something like this CalendarDate class. If performance, memory use, or code size are a concern, I'd recommend this CalendarDate implementation; otherwise, use a Temporal.PlainDate polyfill to be more future-compatible.

The benefits of using CalendarDate are:

• It allows you to be explicit in your API design (in your TypeScript types) about whether you're using a calendar date or a timestamp.
• It lets you avoid all kinds of subtle bugs related to timezones and daylight savings time (this is the voice of experience talking!).
• This implementation is very optimized and is typically 6-9x faster than using the native Date or Temporal.PlainDate classes. It represents each date as a single Number so it is also very memory efficient.
• Unlike Date, it is immutable, which makes your code more predictable.

Usage by example

Instantiation:

import { CalendarDate, D } from "https://deno.land/x/calendar_date_deno@v1.1.0/CalendarDate.ts";

// You can construct CalendarDate instances using the included D literal helper:
const someDate = D`2023-08-15`;
// Or using CalendarDate.create()
const otherDate = CalendarDate.create(2023, 9, 27); // Sept. 27, 2023
// Or using CalendarDate.fromString()
const thirdDate = CalendarDate.fromString("2023-10-02"); // ISO 8601 format
// Or from a JavaScript Date
const convertedDate = CalendarDate.fromDate(new Date("2023-11-12"));
// Or get the current date
const today = CalendarDate.today();

Printing/conversion:

// You can print dates using ISO 8601 format:
someDate.toString(); // "2023-08-15"
// Or get various properties:
[someDate.year, someDate.month, someDate.day]; // [ 2023, 8, 15 ]
// Or convert back to a JavaScript date:
someDate.toDate(); // Date [2023-08-15T00:00:00.000Z]
// Or format using any locale you want (see FAQ):
someDate.format(myLocaleFormat); // "Aug 15, 2023", "15 авг. 2023 г.", etc.

Manipulation:

// You can add days (this returns a new instance; CalendarDates are immutable)
const nextDay = someDate.addDays(1);
// Or add months
const nextMonth = someDate.addMonths(1);
// Or add years
const nextYear = someDate.addYears(1);

For more usage details and examples, just check out the code or the test cases. It's very readable.

Benchmarks

See the included CalendarDate.bench.ts file for details. You can run these benchmarks using deno bench.

Combined features

This test uses fromString() to construct a calendar date, then addDays() to construct a second date, then uses toString() to print both dates in ISO 8601 format.

On this test, CalendarDate is:

• 6x faster than the native Date object
• 9x faster than the native Temporal.PlainDate object.
• 8x faster than calendar-date on NPM
• 18x faster than Day.js
• 25x faster than the pollyfilled Temporal.PlainDate object.

Parsing

This test parses 16 ISO 8601 date strings, as you might do when consuming a JSON API response.

On this test, CalendarDate is:

• 1.25x faster than the native Date object
• 4x faster than the native Temporal.PlainDate object.
• 6x faster than calendar-date on NPM
• 6x faster than Day.js
• 13x faster than the pollyfilled Temporal.PlainDate object.

Iterate throught a year

This test starts with a January 1 date then iterates through every date in the year, converting each date to an ISO 8601 string.

On this test, CalendarDate is:

• 9x faster than the native Date object
• 18x faster than the native Temporal.PlainDate object.
• 20x faster than calendar-date on NPM
• 35x faster than Day.js
• 52x faster than the pollyfilled Temporal.PlainDate object.

FAQ

Q: How do I compute the age of something/someone?

A: Using fullYearsSince().

const today = CalendarDate.today();
today.fullYearsSince(birthDate); // This will print the person's age in years

Q: How do I format a date as a nice string in the user's locale?

A: First, declare a formatter that specifies the user's locale and the "style" of date that you want to use (e.g. medium). Then use CalendarDate's .format() method.

const format = new Intl.DateTimeFormat("en", {
  timeZone: "UTC", // Using UTC is required
  dateStyle: "medium", // "full" | "long" | "medium" | "short"
});

const dateValue = D`2023-08-15`; // a CalendarDate
dateValue.format(format);
// "Aug 15, 2023"

To help you avoid bugs, the .format() method will throw an error if your DateTimeFormat is not using UTC timezone.

In a React application, you can achieve the same effect using react-intl's <FormattedDate>.

Q: How do I format a date range in the user's locale?

A: You'll have to use the formatter yourself to format a range:

const formatter = new Intl.DateTimeFormat("en", {
  timeZone: "UTC", // Using UTC is required
  dateStyle: "long", // "full" | "long" | "medium" | "short"
});

const start = D`2023-07-13`; // a CalendarDate
const end = D`2023-07-27`; // a CalendarDate
formatter.formatRange(start.toDate(), end.toDate());
// "July 13 – 27, 2023"

In a React application, you can achieve the same effect using react-intl's <FormattedDateTimeRange>.

History

This is based on my own PDate and later VDate code. If you happen to need Neo4j compatibility, check out VDate instead.

License

MIT