Group Selection Formula in Crystal Report

[nikhilshah01]

Group ID 1
name, tool, history_action.timestamp
name, tool, history_action.timestamp

Group ID 2
name, tool, history_action.timestamp
name, tool, history_action.timestamp

I need to publish a timestamp_diff for each group of records. I am not sure how to use a group selection formula to do this or is there a different way to do this.
Basically I need the two timestamps in two variables, apply a formula (which I already have written down) and then print the output of the formula for each group of records.

Basically it's finding difference between same columns(data) of two different rows(records).

There are many built in functions to calcualte diff between two columns of the same row (record), but not sure about this. Any help would be welcome. Thanks.