You can't really group on a DateTime field instantly, there are plenty of date/time functions for aggregates. By which you can group by section of the DateTime value. For example, here's a query that'll show the number of account records created on each date.
select day_only(createdDate) createdonDate,
group by day_only(createdDate)
order by day_only(createdDate) desc
Note: day_only() is used to return the date part of a DateTime field.