In that case use below formulas instead:
Create a variable say year
=RelativeDate(ToDate("01/01/"+FormatNumber(Substr([Bex_Date_field];4;4);"0000");"MM/dd/yyyy");ToNumber(Substr([Bex_Date_field]))-1)*7)
Create another variable say StartDate:
=If(DayNumberOfYear([year])=1;[year];RelativeDate([year];-DayNumberOfWeek([year])+1))
Put the Bex_Date_field name and if required, convert it to string.
-Ankush