| EAA-dev Home |     How it Works  Many facts are true only for
a certain period of time. So an obvious way to describe these
facts is to mark them with a period of time. For many that
period is a pair of dates, however Range can be used here to
make that date range an object.  Once defined, effectivity
ranges are then used in queries to return the appropriate
objects that are effective on a certain date.  The effectivity
range if often updated directly, however it usually makes sense
to provide an interface that better fits the need of the class.
A creation method can take the start date of the effectivity
period, and then use an open-ended Range to indicate there is no
end date: this fits cases where something is set to be created
on a certain date and is effective until further notice. When
that further notice comes, you can have a method that indicates
that the object is no longer effective and the date that occurs
Using the employment example suggested in the sketch, we have a
person, Wellington. On Dec