pjb,
If the Unix method works for you then that's fine. However, you should realise that the week number of the year is likely to be governed by the set of business rules your organisation works with - it's not a fixed calendar thing like months & weekdays. There are some guidelines on how to calculate the week number in the Progress Knowledgebase (Kbase #18669) and in the sample code that comes with your installation of Progress ($DLC/src/samples/weeknum.p).