Does your period start the same time every month or is it normal to start the night before on birth control?

MY periods always end at the same place--after the sentence. YOUR periods can start at the same time each month or at (potentially widely) varying times each month. My lover when I was in college once went over 100 days with no period at all. We briefly thought she was pregnant. Then the menstruation started. Her doctor seriously considered putting her on the pill, only to regulate her very irregular periods. If you're on the pill, that schedule WILL regulate your periods. I wouldn't worry about a 12 hour difference; but if you're worried, ask your doctor, not any of the non-M.D.'s here.

Yes, if your cycle has been normal ad no abnormal menses. The other part of your question re: startig the night before on BC: If the BC is new and your body isn't used to it, yes it can be late or early a few times until the cycles level out. If you don't want babies and your menses is off, may I suggest using Safe Sex practices until normal. Keep track with a personal calendar marking your days beginning and ending-mark whether light or heavy flow and mention bright bleeding or mixed wih darker blood. Dr. will help you come up with a care plan for your benefit. Good luck.