When a doctor asks for the last period date, they are typically referring to the first day of your last menstrual cycle. This is important information for them to determine your due date if you are pregnant or to assess any irregularities in your menstrual cycle. Therefore, it is best to provide the start date of your last period when asked by a doctor. However, if you are unsure about the exact start date, providing an estimate or mentioning both the start and end dates can also be helpful for them to get a better understanding of your menstrual cycle pattern.