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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, resolve it with timeZone and disambiguation, then shift the zoned value.

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

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