Do not force the book’s mundhu yawmāni into an ordinary preposition+genitive pattern. One classical analysis treats mudh/mundhu and the duration as a nominal predication; the source’s principal point is the semantic contrast.
Book example: مَا رَأَيْتُهُ مُنْذُ يَوْمَانِ.
I have not seen him for two days.
Mā: negator; raʾaytuhu: verb with subject and object; mundhu yawmāni: duration expression; yawmāni is nominative by alif, analysed as a predicate in a classical construction.