I entered the wrong period date.
If you entered a wrong period start date, go to your period log page (3rd tab). Then click on the wrongly entered start date to edit it.
If you entered a wrong period end date, go to your calendar page (2nd tab). Double-click the wrongly entered end date. Then turn “off” the line that reads “period ended today”. Then double-click the correct end date and turn “on” the line that reads “period ended today”.