In the iOS app, in “Date & Narrative”, you have the choice between year, month, day or time. But in the Mac desktop version, it seems you cannot choose “day” without having the time too, unless I missed something.
I found a way around by erasing manually the time part of the day selection, which is fortunately possible, but it would be simpler to have the same kind of choice as the one in the iOS app, namely day without having to specify a time.