"...as much as possible."

Good, so they left you a little room to work with. I mean "as much as possible" isn't as restrictive as "none at all, ever, for any reason."

Date manipulation is one of the situations where modules are invaluable. Hand rolled solutions usually miss something. Do they want date bugs, or date modules?

Perhaps you could ask your client what date modules they already have installed, and work using one of those.