Forum OpenACS Q&A: Response to date_add, returns "wrong" dates
I hope you realized that I didn't write that function. In fact, I only have a few functions in the cookbook. They are functions contributed by other people. You should write the author about it and point him to this thread, in the spirit of free software.
I'm going to "port" my personal site do OpenACS 4.5 and re-write the cookbook, probably as an ETP application so submitters can update their functions and other people can submit improvements/comments.