YouCanBookMe

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Build me a booking page that replaces YouCanBookMe: availability from my calendar, a slot picked by somebody else, and the meeting in both calendars.

STACK
- Node 20+ with Fastify, server-rendered HTML with a little vanilla JS
- SQLite through better-sqlite3, WAL mode
- Caddy in front

THE DATA MODEL
- users: id, name, slug, email, timezone
- event_types: id, user_id, slug, title, description_md, duration_minutes, buffer_before, buffer_after, min_notice_minutes, max_days_ahead, max_per_day, location_kind, location_value, requires_confirmation, is_active
- schedules and availability_rules: weekday, start_local, end_local, with a named zone
- date_overrides: id, user_id, date, is_unavailable, start_local, end_local
- bookings: id, event_type_id, uid, starts_at_utc, ends_at_utc, attendee_name, attendee_email, attendee_timezone, answers_json, status, cancel_token, reschedule_token, external_event_id, created_at
- calendar_connections: id, user_id, provider, credentials_encrypted, calendar_ids_json, write_calendar_id, sync_token, last_synced_at
- busy_blocks: id, connection_id, starts_at_utc, ends_at_utc, source_event_id, is_all_day, transparency, fetched_at — intervals only, never event details
- reminders: id, booking_id, kind, scheduled_for, sent_at, error

TIME, WHERE THIS GOES WRONG
- Every instant in UTC. Availability rules as local times with a named zone, because 09:00 in Europe/Rome is a different instant in July and in January
- A real time zone library and the IANA database. Never an offset
- The test suite includes a spring-forward day, an autumn-back day, and a zone whose rules changed
- All-day events are dates, not midnight instants — the classic bug in this category

CALENDARS
- Two-way with the major providers, read-only ICS for anything else
- Incremental sync with the provider's sync token, plus a full reconcile on a schedule, because incremental sync drifts
- Recurring events expanded properly, with exceptions and moved instances. A wrongly expanded weekly meeting means double bookings for a year
- Declined and tentative events: decide whether they block, per connection, and say so
- Write the booking to a chosen calendar with the attendee invited, and update or delete it when the booking changes

SLOT GENERATION
- One pure function: rules minus busy blocks minus existing bookings with buffers, then notice, horizon and daily cap, rounded to an increment. No database access inside it, tested heavily
- A hold while the form is filled, released after a few minutes; the final insert re-checks availability inside the transaction. The hold is a courtesy, the transaction is the guarantee

BOOKING
- Pick a day, pick a slot, answer the questions, confirm. No account, ever
- Custom questions per event type, validated on the server
- Cancel and reschedule from long random tokens in the email

REMINDERS
- Rows written when the booking is created and rewritten if it moves, at fixed offsets
- Email always, and SMS if you have a provider — that is a per-message cost and a registered sender in most countries, so decide deliberately
- A confirm-or-cancel link in the reminder, so a cancellation reaches you instead of an empty slot
- Every send recorded, so nobody is reminded twice

OPERATIONS
- .env: DATABASE_PATH, BASE_URL, ENCRYPTION_KEY, SESSION_SECRET, SMTP_URL, provider client ids and secrets
- Migrations on boot, each once; nightly backup off the machine, restore script
- Health endpoint that generates tomorrow's slots and reports sync age

WHAT MATTERS MOST
Recurrence expansion and the booking transaction. Write the time zone tests before the interface exists, then have two browsers book the same slot at once and confirm exactly one succeeds.