“The only reason for time is so that everything doesn’t happen at once.” Albert Einstein.
Time and Events
Time is what happens between events. Without events there will be no time, and each event introduces its own time-scale.
From the (limited) perspective of system requirements, time is the scale used to position business processes, i.e the execution of business activities. And since systems are meant to support business processes, time must be set by business events before being measured by shared devices.
As a counter intuitive corollary, time is only to run with business: if nothing relevant is meant to happen between two events there is no need of time and both events can be coalesced into a single one. Conversely, if nothing is allowed to happen between two events the system will have to deal with them in real-time.
Like events, time is necessarily local since physical devices must sit on location, possibly mobile ones; once set by some triggering event, time can only be measured by a single clock and therefore cannot be shared across distributed systems.
Moreover, along with its measuring device, time is also an artifact built to meet specific concerns. That can be very useful if activities are to be managed in isolation. Like different objects managed within different address spaces, different activities could be time-stamped and controlled by different time scales.
Being set by independent clocks, each built to answer it own concerns, the symbolic representations of activities could then be fenced off so their execution will appear as impervious to external events. Yet, if and when architectural constraints are to be properly identified, execution units will have to be timed along some shared time scale. More generally, distributed activities, being executed along different time scales, would have to be synchronized through a logical clock using its own time scale. Another solution is to synchronize physical clocks using a standard protocol like the Network Time Protocol (NTP).
Along that reasoning, real-time activities can only be supported locally: if events must occur simultaneously within and without the system, i.e nothing is supposed to nothing is supposed to happens between changes in actual objects and their symbolic representation, that can only be achieved under the control of a single system.