The open calendar, task and note space is a mess
Surprised nobody has mentioned this yet, but Org-mode for Emacs [1] is just <i>great</i>, and fits very well to the requirements:<p>- Source of truth: these are text files, so any of git, nextcloud, syncthing etc. will do.<p>- Consistent interface: using emacs might be tough on mobile, but there are some pretty good web interfaces for it [2]. I personally use Orgzly [3] and syntching on my Android<p>- Standard protocols: custom scripting does anything. ical is pretty easy to handle, not sure about webdav.<p>- FOSS: check<p>- Multiple calendars: yep, via Org agenda [4]<p>- Subtask support: As deep as you can go<p>- Custom logic: via emacs scripts (or some creativity if you're using other clients)<p>- Markdown notes: yes, minimal differences between org mode and markdown<p>[1] <a href="https://orgmode.org/" rel="nofollow">https://orgmode.org/</a><p>[2] <a href="https://github.com/DanielDe/org-web" rel="nofollow">https://github.com/DanielDe/org-web</a><p>[3] <a href="https://www.orgzly.com" rel="nofollow">https://www.orgzly.com</a><p>[4] <a href="https://orgmode.org/manual/Agenda-Views.html" rel="nofollow">https://orgmode.org/manual/Agenda-Views.html</a>