How about creating a calculated field based on the datediff in minutes between the recordcreatedate and a constant (say today's date, 12AM) and divided by 15?
Interval = DateDiff(n, DateConstant, RecordCreateDate)/15
This will return an expression of the time of day a record was created in 15 minute intervals (1,2,3,etc.). Then just group by your calculated field.
Russ Conway
Ardent Development
ardentdevelopment@comcast.net