DST transitions handled; gap or overlap slots are skipped or disambiguated

DST transitions create two temporal anomalies that break naive slot generators: a spring-forward gap (2:00–2:59 AM does not exist) and a fall-back overlap (1:00–1:59 AM occurs twice). CWE-682 (Incorrect Calculation) and ISO 25010 functional correctness both apply. A slot generator that iterates wall-clock minutes without DST awareness will produce bookable slots at 2:30 AM on the spring transition — a time that literally does not exist in that timezone. When a customer confirms such a booking, the confirmation timestamp resolves to an ambiguous or invalid time, causing silent downstream errors in reminders, calendar exports, and host notifications.

Generate slots in UTC and convert to the display timezone using an IANA library rather than iterating local wall-clock minutes. IANA libraries skip spring-gap times automatically.

// src/lib/availability.ts
import { toZonedTime } from 'date-fns-tz'

export function generateSlots(date: Date, tz: string, stepMinutes = 30): Date[] {
  const slots: Date[] = []
  const startUtc = new Date(Date.UTC(date.getFullYear(), date.getMonth(), date.getDate()))

  for (let i = 0; i < 24 * 60; i += stepMinutes) {
    const utcSlot = new Date(startUtc.getTime() + i * 60_000)
    try {
      const zoned = toZonedTime(utcSlot, tz)
      slots.push(zoned)
    } catch {
      // Non-existent DST gap time — skip
    }
  }
  return slots
}

For fall-back overlaps, store the UTC value rather than the local wall-clock time so the two 1:30 AM slots are unambiguous in the database.