For marketing purposes, the end of one month equals the next month. Today is September 30, and in the past three days I've received five marketing e-newsletters dated "September." "Yikes," I think to myself, "these folks do run late and are desperately playing catch-up." But if they'd changed the date on the masthead to October, I'd think, "Congrats, these folks are on top of things by issuing their newsletter a few days early."
So what if changing the date means skipping a month? Get out in front of your marketing instead of falling far behind it.
This advice applies in spades to corporate history. History doesn't happen overnight and it can't be reconstructed overnight. Business anniversary books, websites, timelines, and campaigns take time to develop. When your organization has a milestone coming up in a year or two or three, allow plenty of lead time. Your project will go more smoothly and it will cost less.