At the time this step executes in your loop:
Set Variable [$DateNewWeek ; value: Turnus::Dato_Temp + 7 ]
The value in Dato_Temp is zero. It does not appear to have global storage and this is not a field that you need anyway.
Do the "+ 7" entirely in terms of your variable instead.
Use these steps inside your loop:
Set Variable [$DateWeek1 ; value: $DateWeek1 + 7 ]
Set Field [Turnus::Dato_Start ; $DateWeek1 ]
Thanks a lot
As usual, I make things more complicated than it has to be :-)