Calculate time based on criteria

[losdamianos]

Hello

I have a spreadsheet with task list
A B C D E F
name task desc time date time spent

Each person scan his card (name, task ,desc ) time and date is calculated automaticly.
Im trying to calculate time spent on each task So when Person A appear again, in previous record for Person a time spent will be calculated ( new record column E - previous record column E )
column f has to find NEXT record with that name and calculate time difference

please see attached
http://dl.dropbox.com/u/8597042/example.xls