Effective Date

Separately record the date on which the system should act as if a version is valid.

**Problem:** When you find out a fact is not necessarily when you would like to act like that fact is true.

How do you represent the difference between when something you discover a fact and when you would like the system to act like the fact was true?

**Therefore:** Add an effective date to the Perspective. Take the effective date into consideration when computing visibility.