This design uses a calculated field to get the difference between two time fields in minutes and then shows those minute aggregrated within a table group.<br />
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The computed column looks like below where I had to divide by 1000 to get from milleseconds to seconds, and then divide by 60 again to get the number of minutes elapsed.
(row["stop"].getTime() - row["start"].getTime())/1000/60
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This example was created with BIRT 2.3.0
