A due date is calculated from the last menstruation date, not from the date of conception. The reason we do this in the medical field is because the last menstrual date can be determined precisely, whereas that date of conception can only be assumed. A due date doesn’t mean that delivery must occur on a specific date, either. The difference between the calculated due date and real delivery date in your case is not very big (only 4 days). The conception date can be calculated knowing the date of your last menstrual cycle and the cycle duration. It seems likely that you conceived sometime in May of 2006, probably in the second half of that month.