Datediff issue

Hi All,

Im really hoping someone can help me here.

I'm reporting on a date column within a table to capture the time difference between rows for each status change. I'm using the following formula Datediff('n',{AUDIT.CREATEDATE},Next({AUDIT.CREATEDATE})) which is working as expected but as the formula is within the details section it will continue down through the entire report when I need to be able to only report on the grouped data and for it to then reset back to zero after each grouped record. I have attached screenshots to hopefully detail what I have and where it is going wrong.