A conditional access method and apparatus for use with a system for controlling of digital TV program start time. Subscriber access is authorized to digital program streams. Content are divided into a plurality of working periods. Each working period of the plurality of working periods is scrambled with a different working key. A working key of a given period is delivered ahead of the given period. The working key is synchronized with scrambling control bits in an header. working keys are inserted into an entitlement control message (ECM) packet, and the ECM packet is encrypted using a service key. A time period is inserted into the ECM packet during which the service key is valid. A service key is inserted in an entitlement.