Personal mobility means that a subscriber has a personal address or a personal number to which the subscription is linked and which address is completely independent of what terminal currently is meant to be used. The terminal can be connected in any network, e.g. the stationary public telecommunication network, a stationary private network or a cellular network. The network is assumed to have intelligence to be able to route traffic to/from the terminal and at the same time bill the subscription connected to the subscriber's address. This intelligence is often placed in a central node called service node. Examples of telecommunication systems with personal mobility are UPT (Universal Personal Telecommunication) and PCS (Personal Communications Services).
Routing of incoming traffic for terminals or for functions such as e.g. voice message, central message service or barring can be made time independent. The European Telecommunications Standards Institute ETSI calls this function within UPT `Variable Routing`. It is implemented in an SCP (Service Control Point). A personal time table routing incoming traffic for different destinations depending on day of week and/or time of day can in this case be specified for each subscriber. The subscriber can also define alternative destinations to be used if, e.g., a call is not answered at the first destination.
If the subscriber, permanently or temporarily, is at a geografical location having a time zone which is different from that of the service node, especially the SCP, the time dependent routing of traffic can be wrong,. The subscriber wants to be able to specify the time table in local time.
Similar problems can arise for time dependent services in a corporate network, e.g. in a VPN (Virtual Private Network), extending over at least two time zones.