SORT CRITERIA by GROUP BY datepart in Views
I couldn't find any way to do this using the options in the View UI. One way to accomplish it, however, might be to add an additional field to your content type that stores only the day/month/year for the date (set the default to 'now' so that you don't have to manually enter the date, and set the field to 'hidden' on the display options), and then you could use that field to group by in the view.