Mark a period
On the Today screen, tap Started on the first day and Ended on the last. Everything in between is counted as a period day — you do not mark each one.
It started today
Tap Started. A line appears at the bottom confirming it, with Undo if you tapped it by mistake.
The period now stays open. Tomorrow the tile reads Ended, and the day counter on the card starts from day 1 of the new cycle.
It started on another day
The Started tile writes today straight away. For another day, use Calendar → Edit periods or Today → Past period. The cycle card also offers On another day, but only once the cycle has run past every one you have finished — that is when it asks “Has your period started?”. Or add the day in Calendar → Edit periods. You cannot pick a day in the future: a period that has not happened is not a fact yet.
It has ended
Tap Ended and today is written, with an Undo line if you tapped it by mistake. If you forgot for a day or two, the app's own question — “Did your period end?” — carries On an earlier day as its second button. It asks only when the open period has already lasted longer than any finished period you have logged, never on a day inside your own usual length.
A period you forgot entirely
Tap Past period on the cycle card, or open the Calendar and use Edit periods. Give the start and the end; if you only remember roughly, leave Dates are approximate on. Approximate dates are kept as approximate and are never written to Apple Health.
Several past periods at once
In onboarding the last step takes them one at a time: the day your last period started, then Add another for earlier ones. Afterwards, Today → Past period and Calendar → Edit periods do the same job. In onboarding you can give a start and nothing else, and the app keeps it as a start with unknown length — it will count the days between them as cycles, and leave length out of anything that needs it, instead of guessing.
What happens next
One completed length is enough for the line to appear on Today; from two it shows a range — “Your cycles: 31–52 days · last 6”, where the six is how many cycles were counted altogether. That sentence is your own history and nothing else — see how we count.
If you got it wrong
Nothing is fixed. See fix a day you got wrong: periods can be moved, shortened, deleted and put back.