It might be intentional or a copy+paste thing, the javadoc of FOREVER reads:
"... The estimated duration of the era is artificially defined as the
largest duration supported by Duration."
Since this fragment of the sentence is also describing ERA i suspect
its a copy+paste problem, it can be reworded in:
The estimated duration of this unit is artificially defined as the
largest duration supported by @{link Duration}.
Greetings
Bernd
