Formulas and Groups

I have been working on a report which tallies incoming clals and wait times for my company. Basically we are looking for a count of calls under a thirty second wait time, from 30 seconds - 1 minute wait time, 1 minute - 1 1/2 minutes etc...
I have the formula working correctly however I cannot figure out how to reset the count after it goes into the next time segment as we are reporting on this for every 30 minutes of the day. for example 7:OO Am is working correctly reporting that there were 11 calls answered under thirty seconds, however past this it will keep incrementing in the &:30 detail line section (for this I will be suppressing detail lines and just looking at the summary totals in the group footer) I tried setting it to have the variable reset to zero if the time changes but all this managed to do was have a result of zero returned across the board. Has anyone any experience in this? Thanks

If you are grouping on the half hour you can set your running sum variable
to reset on change of group, which is probably what you want.

HTH