@ceptesoft/tz-day

Timezone-correct calendar day (YYYY-MM-DD). Zero dependencies.

new Date().toISOString().slice(0, 10) gives you the UTC day. For a business in Istanbul (UTC+3), that answer is wrong every night between midnight and 3 AM local — date-keyed orders and route plans land on yesterday. Extracted from production ERP code (CepteCari) where this exact off-by-one corrupted planning days.

Scope boundary: calendar-day identity only — no date arithmetic, formatting, or parsing (use Temporal / date-fns for that).

Install

npm install @ceptesoft/tz-day

Quickstart

import { isoDayInTimeZone, createTodayFn } from "@ceptesoft/tz-day";

// Ad hoc
isoDayInTimeZone("Europe/Istanbul");                                  // e.g. "2026-07-09"
isoDayInTimeZone("Europe/Istanbul", new Date("2025-06-15T21:01:00Z")); // "2025-06-16"

// App-wide: bind the business timezone once
const today = createTodayFn("Europe/Istanbul");
today(); // "2026-07-09" — same answer on server and client

API

isoDayInTimeZone(timeZone: string, instant?: Date): string

The YYYY-MM-DD day of instant (default: now) in the given IANA timezone. Throws RangeError on invalid identifiers.

createTodayFn(timeZone: string): () => string

Binds a timezone once and validates it eagerly (typos throw at startup, not at 3 AM in production).