You linked to the rhythm method, not fertility awareness. Fertility awareness means actually measuring the physiological changes resulting from the hormonal ch

You linked to the rhythm method, not fertility awareness. Fertility awareness means actually measuring the physiological changes resulting from the hormonal changes that precede ovulation (temperature, saliva, cervical mucous). This is much more reliable than guessing based on the date, which is obviously going to fail relatively often as ovulation isn't reliably regular even in the most "regular" cycles.