CONNX has four functions to determine which week a date occurs in:
WEEK(timestamp) for ISO week
WEEKE1(timestamp) for Excel WEEKNUM(CELL,1)
WEEKE2(timestamp) for Excel WEEKNUM(CELL,2)
WEEKS(timestamp) for simple week numbers.
Depending on how you want to calculate which week a date occurs in, one of these functions should satisfy your needs.
Here is an example of how the four functions calculate specific week numbers: