Hello sjang
SBB is aware of the fact that there is a technical glitch in the SBB web ticketing system which may result in an alteration of the date of birth (day); this rarely happens, and if, only for customers booking from another time zone.
I can assure you, all ticket inspectors have been made aware of this error and will accept a ticket whose date of birth is slightly different. It does not mean, however, the ticket is accepted if someone intentionally entered a wrong date of birth to benefit from discounts (e.g. child fare instead of adult fare).
Kind regards,
Roland