You can set the start date of your period by tapping the "my period started" button, a big purple button in the top middle of your calendar, and indicate the exact date it started.
Note: Do not tap the "undo" button (it's a circle with an arrow in the top right of your calendar) to schedule a new period. The undo button is for rescheduling your previous period.
You can change the start date of your period anytime by tapping the "undo" icon in the top right corner of your calendar, just to the left of the "+" icon. This icon looks like a circle with an arrow on it. Enter your period start date in the data toggle that appears and tap "save". Here’s a quick video tutorial to show you how:
Note: You cannot schedule your start date for a day in the future. If your period hasn't started yet, but you've accidentally indicated that it has, just wait until it does start and then use the "undo" button to schedule the start date for that day.