I have an annual season ticket. The expiry date is shown in large bold text, but no start date is shown. The validity (12 months) is shown. Many websites state that season tickets can only be purchased in advance the previous afternoon, or Friday if due to start Monday. I am wondering why this is. Is this because season tickets work in barriers before the start date i.e. once they are printed? Obviously, a ticket is not valid for travel before the start date.