Calender questions!!!

When it asks for period cycle like "how long between each period if it 28 days or 30". Is it from the day you start your period to the day you start your period the next month or is it the day you end your period to the day you start your period next month?? Just a little confused. Because I always count since I end my period to the next month I start which is usually 28 but I dont know if im doing it wrong :( Please let me know your opinion!