That is false. The calendar method was around 50 years ago or so. Currently taught methods of Natural Family Planning include the Billings Ovulation Method, the Creighton Method, and the Sympto-Thermal Method. All of these involve the daily observation of one or several signs of fertility, and the teaching of all of them cautions couples who have serious reasons to avoid conceiving NOT to rely on cycle length as a sign, EVER.
If a website that's not simply presenting a historical curiousity is discussing Calendar Rhythm, I suspect a deliberate intention to conceal the effective methods of fertility awareness from potential readers.
http://www.ccli.org/
Any errors in my post were not intentional. I was just trying to point out that natural family planning methods are just as effective in real world use as birth control pills. In addition they are less harmful, and cheaper as well.
My apologies for any terminology misuse.