Is your date '[week_min_date]' a date field or a text field. I see, you are trying to use date() around the date field in your set expression. Check in the list box, if the date field is left-aligned or right-aligned. If right-aligned, the following expression style should work.
If left-aligned, it's a text field, you have to correct it in the script like:
Date(Date#([week_min_date], 'format')) as [week_min_date]