Vielzeug

DST-Safe Arithmetic ​

Problem ​

A wall-clock value during a DST transition is not an instant until you choose a timezone and disambiguation strategy.

Solution ​

Parse the wall-clock value explicitly, then use shift() with timeZone and disambiguation for DST-safe arithmetic.

ts
import { parse, shift } from '@vielzeug/tempo';

const wallClock = parse('2026-11-01T01:30:00', { as: 'plainDateTime' });
const nextHour = shift(wallClock, { hours: 1 }, {
  disambiguation: 'later',
  timeZone: 'America/New_York',
});

Pitfalls ​

  • Pass timeZone for every plain date or plain date-time conversion.
  • Choose disambiguation: 'reject' when ambiguous local input must fail.