Day Boundary Library

Assign timestamps to operational windows when midnight breaks your logic.

day-boundary resolves DST-safe [start, end) windows from explicit local boundary rules such as "09:00 in Europe/London". Use it when your reporting day, service window, shift cycle, or operational cutoff does not align with midnight.

This README covers the current 3.x package line.

Start here

If you are installing this package from npm:

1. Install Node 20+.
2. Use an ESM project. This package is ESM-only.
3. Run npm install day-boundary.
4. Use Temporal inputs, not legacy Date, string timestamps, or numeric timestamps.
5. Copy the quick-start example below.

Golden path

For most applications, do this:

1. Define a boundary strategy.
2. Resolve the [start, end) operational window for an exact timestamp.
3. Use window.start and window.end for querying, grouping, and reporting.

If you only use one function, start with getWindowForInstant(...).

Quick start

import { Temporal } from '@js-temporal/polyfill';
import { FixedTimeBoundaryStrategy, getWindowForInstant } from 'day-boundary';

const strategy = new FixedTimeBoundaryStrategy({
  timeZone: 'Europe/London',
  boundaryTime: '09:00',
});

const window = getWindowForInstant(Temporal.Now.instant(), strategy);

console.log(window.start.toString());
console.log(window.end.toString());

Start with FixedTimeBoundaryStrategy and getWindowForInstant unless you already have a more specific input shape.

This package is ESM-only. Use import ... from 'day-boundary', not require(...). The typed API is Temporal-only. Do not pass legacy Date, string timestamps, or numeric timestamps.

Critical DST note

This library is designed for systems where DST correctness matters.

The boundary resolution APIs handle DST transitions correctly, which means a window is not always 24 hours long. Depending on the transition, a resolved window may be 23, 24, or 25 hours.

Also keep these two rules separate:

• elapsed duration: actual time passed
• wall-clock duration: local scheduled clock time

On DST transition days, those can produce different answers.

For the full explanation and examples, see DST and duration semantics.

What it solves

Use this library when:

• your operational day starts at a non-midnight boundary
• events cross midnight and still belong to one business window
• reporting or grouping must follow operational windows, not calendar dates
• DST and non-24-hour days must be handled correctly

Avoid it when your system is strictly calendar-day based and midnight is already the correct boundary.

What stays out of core

day-boundary resolves boundary-defined time windows correctly.

It does not encode business policy such as attendance, payroll, shift labeling, SLA status, reporting labels, or ETL workflow rules. Those belong in downstream applications and companion packages built on top of the core boundary primitive.

This keeps the library small, neutral, and trustworthy across domains.

Installation

Requires Node 20 or newer and is ESM-only.

npm install day-boundary

The package includes @js-temporal/polyfill as a dependency.

TypeScript consumers get a strict declaration file, and the typed API is Temporal-only.

Choose the right entry point

• getWindowForInstant for exact timestamps and most server-side use
• getWindowForZonedDateTime when you already have a zoned Temporal value
• getWindowForPlainDateTime when the user enters local clock time

For larger reporting or backfill workloads, see the large-dataset guidance in the usage guide.

Try the demo locally

Published examples site:

https://dayboundary.gazali.one/

If you want to run this repository locally:

npm install
npm test
python -m http.server 8000

Open this first:

http://localhost:8000/examples/day-boundary-operational-day-demo/

That is the best first browser example for understanding the library. The full examples tour lives at:

http://localhost:8000/examples/

Reference CLI

time-window-classifier (twc) is a reference CLI that uses day-boundary to process JSONL event data and compare calendar-day grouping with operational-window grouping.

Main exports

• BoundaryStrategy
• FixedTimeBoundaryStrategy
• DailyBoundaryStrategy
• getWindowForInstant
• getWindowForZonedDateTime
• getWindowForPlainDateTime
• getWindowProgress
• getWindowEndByElapsedDuration
• getWindowEndByWallClockDuration
• compareWindowEndings
• isSameWindow
• groupByWindow
• getWindowId

Window IDs are stable across DST transitions and safe for grouping and persistence.

Read more

• Usage guide
• API guide
• Functions reference
• DST and duration semantics
• v3 migration guide
• SQL DST-safe queries
• Business use cases
• Examples README
• Project wiki

Version note

• day-boundary is the current 3.x root API
• 3.1.5 adds mobile examples-site polish, a clock/boundary favicon, Node 20+ support policy alignment, and Node 20/24 CI coverage with no public API change
• ver-01, ver-02, and ver-03 in this repository are archive folders only
• if you need older published behavior, use day-boundary@2.x

Summary

This is not a general date utility. It is a boundary-window library for systems where a meaningful day starts somewhere other than midnight.