time scale calculation

[#1]

I have a table with three colums

colum one is named  start time

colum two is named end time

colum three is named employee number

now i want to know how many employees where in the office at different times  between 08.00 and 17.00

therefore i use the calculated dimension:

.

=timestamp(class(frac(Tijd_Start) ,30/(24*60)),'hh:mm')

question is which expresion do I have to use

[#2]

Did this get resolved as I'm having a similar problem using the script Marc has posted.

Jim