It is in the SiYuan source code, no dependent plugin or snippet. Also, calender stays calender, it doesnt have "non-calender" features like tasks or pomodoro which are included in one calender plugin.

🎓 Just published my second agent skill: siyuan-agent

It lets AI agents to interact with SiYuan notes. Read and write without middleware, just HTTP. Very handy for a second brain workflow.

It works as a CLI, so both human users and AI agents can use it.

Check it out: https://github.com/eloklam/siyuan-agent