appdate

A timezone-aware date and time abstraction built on Day.js.

Installation

npm install @ma.vu/appdate
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Development

bun install
bun run demo.ts
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Initialize and compare dates

Use initializeAppDate as the entry point for language, timezone, and working-day configuration. workingDays uses 0 for Sunday through 6 for Saturday and defaults to Monday through Friday.

import { AppDate, initializeAppDate } from "@ma.vu/appdate";

await initializeAppDate({
  language: "en",
  timeZone: "Europe/Zurich",
  workingDays: [0, 1, 2, 3, 4], // Sunday through Thursday
});

const start = AppDate.fromUtcString("2026-01-13T10:30:00Z");
const end = AppDate.fromUtcString("2026-01-15T09:30:00Z");

end.diff(start, "day"); // 1 (truncated)
end.diff(start, "day", true); // 1.9583...
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Formatting

AppDate uses Day.js format tokens; foreign tokens such as EEEE and yyyy throw, and literal text must be wrapped in square brackets, such as [EEEE].

Calling format() without a template returns an ISO-compatible timestamp with second precision, such as 2026-07-15T17:00:00+02:00.

Migrating from 0.x

setTimezone and setAppDateLanguage were removed in favor of initializeAppDate. Before initialization, AppDate now uses the system timezone instead of Europe/Zurich, falling back to UTC when the runtime cannot report a system zone.

How?

The pattern follows the Builder/Factory pattern where:

Static methods are the "builders" that create new instances

// These create NEW instances of AppDate
static now(): AppDate
static fromDateString(date: string): AppDate
static fromLocalTime(time: string): AppDate
static invalid(): AppDate
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Instance methods are the "operators" that work with existing instances

// These operate on an EXISTING AppDate instance
add(value: number, unit?: ManipulateType): AppDate
subtract(value: number, unit?: ManipulateType): AppDate
isValid(): boolean
toLocalTime(): string
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Why Constructor is private?

The constructor is marked as private for several important reasons:

It ensures all date creation goes through the static factory methods where proper validation happens It prevents creating AppDate with invalid/unexpected formats It maintains a single way to create dates, making the code more predictable It encapsulates the internal dayjs implementation

If we allowed direct construction:

// If constructor was public:
const date1 = new AppDate("2024-02-11"); // Is this YYYY-MM-DD?
const date2 = new AppDate("02/11/2024"); // What about this format?
const date3 = new AppDate("11.02.2024"); // Or this European format?
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Instead, with factory methods:

// Clear intention, validated format
const date = AppDate.fromDateString("2024-02-11"); // Must be YYYY-MM-DD
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