How the dates are predicted
Each period is expected one cycle length after the last, and each fertile window sits five days before to one day after ovulation, which is 14 days before the following period. The cycle phases shown are: Period, Follicular (period to fertile window), Fertile window, and Luteal (after ovulation until the next period).
Common questions
How does it know what phase I'm in today?
It compares today's date to your cycle start and typical length — the days up to your period length are "Period", up to the fertile window is "Follicular", then the fertile window itself, then "Luteal" until the next expected period.
Can it predict irregular cycles accurately?
Less well — the whole projection assumes your entered cycle length repeats consistently. If your cycles vary by more than a few days month to month, treat predictions more than one cycle out as rough estimates.
For general information only — not medical advice. Talk to a doctor about anything that concerns you.