The menstrual cycle starts on day one of the bleeding to day one of the next. On average, the cycle lasts for 28 days, but it’s normal to have it longer or shorter. So if one period started on July 10 and the next one started on August 5, then you would have a 26 days cycle. If your periods are regular, then the two subsequent ones would come on August 31 and September 26. Read this article for more information.