Two ways that I know of...

1) In your data table, set up a new column with the funtion
=Round(A1,0) where A1 is the Date&Time field. The "time" is
represented as everything to the right of the decimal place where the
"date" is to the left. This basically converts all of our Date/Time
fields to midnight.

2) In the pivot table, use the Group and Show Detail to convert the
dates/times to "Days". Right click on the list of Date/Times then
choose Group and Show Detail Group. Click on Days.

- John