Nice, and I like the idea that the past is fixed, but ... is there a way to define the point of rollover to the next day? My "days" sometimes end at 0:50 for example and not at 23:59. So I might summarize the day a bit after midnight.

Good idea, I can do that!

If you want to avoid too much choice, but still want the "the past is immutable" feel, you can prevent editing after noon next day or similar.

Days "roll-over" when you sleep, so it should be safe to rollover notes which haven't been edited for 6 hours. That way there is no fixed rollover time.

But then if you edit a note in the morning to add something, by night it'll be locked.

Having the default "midnight" be something like 3AM would get you 90% there without any UI changes

Many lootbox phone games do rollovers at 4AM local. Your sleep cycle would be seriously broken if you're awake 4:00 +/- 0:30

I just use a .org file, with git to retain old versions if I edit something that might be of later relevance.