Group by datetime

I'm doing a Group By on my report and I want to do it by a datetime field. I want it to group by date only. I only have 1 datetime field in my table, no other dates are stored.

However when I set it up and groups it by a date that not even in my table.

It puts 1/17/2010 as the only group which doesn't even exist in my table. Why is it doing this? And how do I get it to group correctly using the dates that are present in a certain date field in the table only? Anyone know the formula for this?

Create a formula field (invisible) that strips to time out (using datepart) and group on that field.