Time dependency appears in our conceptual model implicitly. At each instant only the set of policies which exists at authorization time is considered. All future or past policies are irrelevant. Note that this does not mean that the current policy does not depend on the past or future events. Some policies must take into account the system execution history or the fact that particular event must have happened for some operation to take place. An example of practical policy taking into account occurrence of some event is ``If one reads file 1#1, then one can not send'' . Some policies may need to know precise time of the event occurrence, for example for audit purposes. This may require a time-stamping of certain occurrences and keeping record of them.