Mycelium reads your iMessage history, finds what's time-sensitive, and shows it as a list of dates. You approve or dismiss each one. Nothing reaches your calendar on its own.
Your Messages database never leaves the machine. No account, no server, no network connection at any point.
“Friday at 3” resolves against when the message was sent, not when you opened the app. Expired and undated items stay out.
Approve or dismiss. Dismissals are permanent — rescanning never resurrects something you already ruled out.
macOS keeps Messages private and — unlike Contacts or Photos — it never shows a prompt for it. There is no API that can ask. Mycelium opens the right settings page for you; you flip one switch, once.
That is also why it ships as an app rather than a script: the permission attaches to Mycelium itself, not to your terminal.