Skip Today

If you didn't measure that morning or don't have any other data to add, you can skip today. This means that you're just actively telling the app that you don't have any data to add. You can at any time during the day open up your app again and add new data, such as period or sex. 

This can only be done for Today, and not for previous days.

This is also shown in the Calendar view, with the small arrow indicating the days that you have skipped. Along with the small check marks that shows which days you've added data on.