cal­en­dar­week – Cal­cu­late the week num­ber of a date

Cal­cu­lates the week num­ber of a given date; us­age is \cal­en­dar­week{30}{12}{2006}. The pack­age uses the e-TeX ex­ten­sions (avail­able in any cur­rent dis­tri­bu­tion); it is suit­able for in­clu­sion into larger cal­en­dar-based pack­ages.
Watch out: Dec 29 can be eval­u­ated to week 1 of the next year; and Jan 3 can be week 53 of the pre­vi­ous year.