The time reference of a user terminal that is derived from an unauthenticated access point can be certified by a trusted entity according to the present invention. In one embodiment, the present invention includes a user terminal obtaining a time reference from an access point of a wireless access network. The user terminal then receives a digital certificate having a validity period from the access point, the certificate being issued by a certificate authority. In one embodiment, the user terminal requests certification of the time reference by a trusted entity, and receives the certification. Then the user terminal can validate the digital certificate.