zooming to a day should convert the day to segments: 1/2 day to 1/4 day to 1 hr to 15 min
The zoom metaphor breaks down as you zoom in on a specific day.
As I zoom to a day, the days should convert to segments, where I can create a new entry that spans some number of segments.
The segments should change with the zoom, starting, perhaps, with day / night, then changing to morning / afternoon / evening / late night, then hours, then 15 minute segments. At any point, I should be able to highlight a range and add a new entry.