Alternatives
Thunderbird Appointment
Open-source scheduling from the Thunderbird project, built on open standards and self-hostable.
Thunderbird Appointment is an open-source scheduling tool from the Thunderbird project, released under the MPL-2.0 license, that lets you publish customizable availability and share booking links so others can grab time on your calendar. It works as a mobile-friendly web app and connects to calendars through the open CalDAV standard as well as Google and Microsoft, with Zoom integration for meeting links and real-time RSVP tracking.
Its pitch against Calendly is architectural rather than feature-based: open standards instead of proprietary lock-in, a privacy-focused steward instead of an ad-adjacent SaaS vendor, and the option to self-host the entire stack via Docker at no license cost.
The important caveat is maturity. Appointment has been absorbed into Thunderbird's paid subscription bundle — renamed Thundermail in June 2026 — alongside the Thundermail email service and the Send file-transfer tool. It remains in invitation-only beta with a waitlist, and there is no free hosted tier yet, though Thunderbird has said it plans to add a limited one. Development is genuinely active, with frequent releases through 2026, but it is not yet a drop-in Calendly replacement: you cannot sign up on demand, and it lacks deeper team scheduling, routing, and payment features.
Privacy-conscious users and open-source advocates who want a self-hostable scheduler built on open standards.