How to get Time Difference between two dates in report viewer 2010

column_A   column_B   column_C

A10.07.2012 17:47:01    0:0:0

A13.07.2012 13:45:03    19:58:01<--here need for 05:58:02

A17.07.2012 10:26:39    20:41:36  <---Here  I need 6:43:38 and using DateDiff()

but i need office working hours only get column_C, and office working time 08:00:00AM and end time 6:00:00PM i'd get column_C values using 

=IIF(Fields!DocumentID.Value=Previous(Fields!DocumentID.Value),DateDiff("H",Previous(Fields!Date.Value),(Fields!Date.Value) mod 24 &":" & Datediff("N",Previous(Fields!Date.Value),Fields!Date.Value) MOD 60 & ":" & (Datediff("S",Previous(Fields!Date.Value),Fields!Date.Value) MOD 60) MOD 60,  "0:0:0")

how to get a person working hours

anybody please help me...

Thanks

Emalai