It depends in part on the datatype of your time field, but if it is a time field, you should be able to create formulas like:
if {table.time} in time(07,00,00) to time(07,30,00) then "7:00 to 7:30" else
if {table.time} in time(07,30,01) to time(08,00,00) then "7:30 to 8:00"//etc.
Then group on this formula and insert summaries on your transaction field. Then suppress the details.
-LB