Tool Profile

Thunderbird Appointment

Open-source scheduling tool positioned as a Cal.com replacement.

Why developers recommend it

A Thunderbird maintainer explicitly pitched it as the open-source replacement path.

Reference ID

thunderbird-appointment

Hacker News Mentions

2026-04-16

“Our scheduling tool, Thunderbird Appointment, will always be open source” and “We’ll help you replace Cal.com”.

Cal.com is going closed source