Timezones and visible ranges

Set the calendar timezone once:

const calendar = useCreateCalendar({
  views: { month: true, week: true },
  timeZone: 'America/New_York',
})

That timezone drives the initial date, today(), isToday, navigation ranges, and local-midnight boundaries. Calendar behavior does not fall back to the host timezone after a timezone is supplied.

Half-open query contract

type VisibleRange = {
  start: Temporal.ZonedDateTime
  end: Temporal.ZonedDateTime
}

Use >= start and < end. Adjacent ranges meet without gaps or duplicate boundary records, and the code never has to invent the final nanosecond of a day.

const { start, end } = calendar.visibleRange

await db.event.findMany({
  where: {
    startsAt: { gte: start.toInstant(), lt: end.toInstant() },
  },
})

DST is calendar geometry

A local calendar day may contain 23, 24, or 25 elapsed hours. Calendar constructs each boundary from its own local midnight rather than adding 24 hours to an instant.