Checking for a week number in a workflow expression

[joebordes]

I think you can make it displaytype 4, those are read-only but updatable from workflows and then hide it from the profile of the user. Try that

Answering your previous question: "Would this also work in reports by the way?" No, we still don't have calculated values in reports but you could follow the same pattern I propose above and create a custom field to hold the calculation to report on.