Google Calendar → Settings → your calendar → Secret address in iCal format. Events fill the Planned column. Anything you’ve typed yourself is never overwritten, and past days are left alone.

A read-only copy of this day as it is right now. Anyone with the link can view it without an account, but can’t change anything. Later edits don’t show until you update the snapshot.

15-minute slots and the Actual column are always there. Switch on only the extras you’ll use.