I can agree with you, however, the length of an interval in a mathematical sense does not depend on inclusiveness if we are talking about intervals over the real numbers, which is a fair assumption unless stated otherwise.

You didnt even specify whether interval start and end are exclusive or inclusive.
Awful Kata description.

Should have added that BIG INT libraries are not allowed, otherwise this kata has no point.
Everyone is just using a library, ignoring the actual point of this kata.