Slotwise: tutor bookings and deposits, designed and built

Solo tutors lose time to messages about when to meet, and money to students who don't turn up. Slotwise gives a tutor one link. Students pick a time shown in their own zone, pay a deposit and get a calendar invite.

I designed it around the way tutors already think about their week: a school timetable. Each subject has its own colour, open times are blocks you can tap, and closed days are hatched so nobody wonders whether they're free. The deposit sits next to the picked time, so the price is never a surprise at checkout.

Two students, one time. If two students book the same time at once, both requests can pass the app's check. So the database decides: a Postgres exclusion constraint rejects any two live bookings that overlap. The second student sees "Someone booked that time a moment ago". A test fires two requests at once to prove it.

Payments that arrive twice, or late. Stripe can send the same webhook more than once, so each event is stored in the same transaction as its effect. A time is held for 10 minutes. A late payment confirms only if the time is still free; otherwise the deposit is refunded automatically.

Time zones and clock changes. An hour skipped when clocks go forward isn't offered, and an hour that happens twice is offered once. Tests pin London, New York, Kolkata and Kathmandu (UTC+5:45).

98 Lighthouse mobile performance and 100 accessibility on the booking page (median of 3 runs, 14 Oct 2026). 156 unit and database tests, plus 18 browser test runs on desktop Chrome and a Pixel 7, green in CI.

Built with Next.js, TypeScript, Postgres on Neon with Drizzle, Auth.js, Stripe Checkout (test mode), Resend, Vitest with PGlite and Playwright, on Vercel.

Live demo: slotwise-booking-xi.vercel.app ("Try the demo" signs you in to a sample tutor). Code: github.com/krishnanbhupathi/slotwise-booking