Although I think your comment here rather underestimates mathematicians :) Regular people are the ones who are only used to thinking in real numbers or integers. While some may be familiar in a practical way with how dates and times work, they would probably struggle to rigorously define the algebra of time math where associativity and commutativity don't hold. Mathematicians will be familiar with areas like abstract algebra and group theory and very capable of understanding the concept that date math is not normal integer arithmetic.
Either way though, I agree the plus operator works great here given the inherent weirdness of how we have structured human time, and everything the author is proposing is worse. Joda handles the trickiness of dealing with time far better and in a much less error-prone way than any other library I've seen.