Group by gives good output and details gives bad output

[jongag1]

I have a report whichs generate data about past day's.
I want to sort the report in weeks and I use a function @groupbyweek:
{STARTDATE}-dayofweek({hdvdn.STARTDATE})+2
to tell Monday is the start date.

The output I get is as follows:

28 october
- 27-10-2002 120
- 28-10-2002 040
- 29-10-2002 640
- 30-10-2002 040
- 31-10-2002 030
- 01-10-2002 120
- 02-10-2002 240

4 november
- 03-11-2002 140
- 04-11-2002 030
- 05-11-2002 045
- 06-11-2002 041
- 07-11-2002 130
- 08-11-2002 060
- 09-11-2002 040

The Group By is ok, but when I look in the details it is not ok and is Sunday the starting day.
Can somebody please help me out?

Kind regards,
Age de Jong