Why does the āyah mention "forty nights" instead of "forty days"?

Imām Ibn al-Jawzī (d. 597/1201) summarized the scholars' opinions in his book "Zād al-Masīr" as follows:

1. The nights are mentioned instead of the days because it is the custom of the Arabs to keep dates according to the night. This is because the beginning of a month is marked by its night, and the Arabs rely on the lunar calendar, as a result of which the days follow the nights (rather than the reverse).
2. Abū Bakr al-Naqqāsh held that the nights were mentioned because Moses was commanded to fast during these days and continue the fast into the nights, so the nights were mentioned. Ibn al-Jawzī comments: This view is baseless.