The adverbial genitive also survives in a number of stock phrases; for example, in "I work days and sleep nights", the words days and nights are analyzed as plural nouns but are in fact derived historically from the genitive or instrumental cases of day and night.
The adverbial genitive is found in several common expressions, such as "I work days and sleep nights".  In these phrases, the words "days" and "nights" are treated as plural nouns, even though they actually derive from the genitive or instrumental cases of "day" and "night."