Hi Gromit,
if you like the german way to calc the calenderweek
you must look to the 4 rules
1) First week with 4 Days
2) First week with a thursday
3) The Week include the 4 January
4) First complete Week
I found this KWeek = ((d - t - 3 + (Weekday(t) + 1) Mod 7)) \ 7 + 1
I hope is helpfull.
Bye Kaljj