Loop that subtract date fields in a report

i want to write a formula that filters time from a dateTime field and subtract it from the record before it. for example

No Date_Time @Formula

A 2/14/2008 03:04:23am A-0 (i.e 03:04:23am -0)
B 2/14/2008 03:05:01am B-A (03:05:01am-03:04:23am )
C 2/14/2008 03:04:44am C-B (03:04:44am-03:05:01am )

what i need to achieve is that, the formula should subtract the first record time from 0 and then subtract subsequent records time from the date field that appears before them
hope this is clear enough.
Thanks